feat: add SwaggerUI and v1 API docs (#2115)

* - Refactor several API routes from anonymous functions to regular definitions. Required to add parseable documentation comments.

- Add API documentation comments using Swag Declarative Comments Format

- Add echo-swagger to serve Swagger-UI at /api/index.html

- Fix error response from extraneous parameter resourceId to relatedMemoId in DELETE("/memo/:memoId/relation/:relatedMemoId/type/:relationType")

- Add an auto-generated ./docs/api/v1.md for quick reference on repo (generated by swagger-markdown)

- Add auxiliary scripts to generate docs.go and swagger.yaml

* fix: golangci-lint errors

* fix: go fmt flag in swag scripts

docs/documenting-the-api.md

@@ -0,0 +1,113 @@
+# Documenting the API
+
+## Principles
+
+- The documentation is generated by [swaggo/swag](https://github.com/swaggo/swag) from comments in the API code.
+
+- Documentation is written using [Declarative Comments Format](https://github.com/swaggo/swag#declarative-comments-format).
+
+- The documentation is generated in the `./api` folder as `docs.go`.
+
+- [echo-swagger](https://github.com/swaggo/echo-swagger) is used to integrate with Echo framework and serve the documentation with [Swagger-UI](https://swagger.io/tools/swagger-ui/) at `http://memos.host:5230/api/index.html`
+
+## Updating the documentation
+
+1. Update or add API-related comments in the code. Make sure to follow the [Declarative Comments Format](https://github.com/swaggo/swag#declarative-comments-format):
+
+   ```go
+   // signIn godoc
+   //
+   //  @Summary    Sign-in to memos.
+   //  @Tags       auth
+   //  @Accept     json
+   //  @Produce    json
+   //  @Param      body    body        SignIn      true    "Sign-in object"
+   //  @Success    200     {object}    store.User  "User information"
+   //  @Failure    400     {object}    nil         "Malformatted signin request"
+   //  @Failure    401     {object}    nil         "Password login is deactivated | Incorrect login credentials, please try again"
+   //  @Failure    403     {object}    nil         "User has been archived with username {username}"
+   //  @Failure    500     {object}    nil         "Failed to find system setting | Failed to unmarshal system setting | Incorrect login credentials, please try again | Failed to generate tokens | Failed to create activity"
+   //  @Router     /api/v1/auth/signin [POST]
+   func (s *APIV1Service) signIn(c echo.Context) error {
+   ...
+   ```
+
+   > Sample from [api/v1/auth.go](https://github.com/usememos/memos/tree/main/api/v1/auth.go)
+   > You can check existing comments at [api/v1](https://github.com/usememos/memos/tree/main/api/v1)
+
+2. Run one of the following provided scripts:
+
+   - Linux: `./scripts/generate-api-documentation.sh` (remember to `chmod +x` the script first)
+   - Windows: `./scripts/generate-api-documentation.ps1`
+
+   > The scripts will install swag if needed (via go install), then run `swag fmt` and `swag init` commands.
+
+3. That's it! The documentation is updated. You can check it at `http://memos.host:5230/api/index.html`
+
+### Extra tips
+
+- If you reference a custom Go struct from outside the API file, use a relative definition, like `store.IdentityProvider`. This works because `./` is passed to swag at `--dir` argument. If swag can't resolve the reference, it will fail.
+
+- If the API grows or you need to reference some type from another location, remember to update ./scripts/generate-api-documentation.cfg file with the new paths.
+
+- It's possible to list multiple errors for the same code using enum-like structs, that will show a proper, spec-conformant model with all entries at Swagger-UI. The drawback is that this approach requires a major refactoring and will add a lot of boilerplate code, as there are inconsistencies between API methods error responses.
+
+  ```go
+  type signInInternalServerError string
+
+  const signInErrorFailedToFindSystemSetting signInInternalServerError = "Failed to find system setting"
+  const signInErrorFailedToUnmarshalSystemSetting signInInternalServerError = "Failed to unmarshal system setting"
+  const signInErrorIncorrectLoginCredentials signInInternalServerError = "Incorrect login credentials, please try again"
+  const signInErrorFailedToGenerateTokens signInInternalServerError = "Failed to generate tokens"
+  const signInErrorFailedToCreateActivity signInInternalServerError = "Failed to create activity"
+
+  type signInUnauthorized string
+
+  const signInErrorPasswordLoginDeactivated signInUnauthorized = "Password login is deactivated"
+  const signInErrorIncorrectCredentials signInUnauthorized = "Incorrect login credentials, please try again"
+
+  // signIn godoc
+  //
+  //  @Summary    Sign-in to memos.
+  //  @Tags       auth
+  //  @Accept     json
+  //  @Produce    json
+  //  @Param      body    body        SignIn      true    "Sign-in object"
+  //  @Success    200     {object}    store.User  "User information"
+  //  @Failure    400     {object}    nil         "Malformatted signin request"
+  //  @Failure    401     {object}    signInUnauthorized
+  //  @Failure    403     {object}    nil "User has been archived with username {username}"
+  //  @Failure    500     {object}    signInInternalServerError
+  //  @Router     /api/v1/auth/signin [POST]
+  func (s *APIV1Service) signIn(c echo.Context) error {
+      ...
+  ```
+
+### Step-by-step (no scripts)
+
+#### Required tools
+
+```bash
+# Swag v1.8.12 or newer
+# Also updates swag if needed
+$ go install github.com/swaggo/swag/cmd/swag@latest
+```
+
+If `$HOME/go/bin` is not in your `PATH`, you can call `swag` directly at `$HOME/go/bin/swag`.
+
+#### Generate the documentation
+
+1. Run `swag fmt` to format the comments
+
+   ```bash
+   swag fmt --dir ./api/v1 && go fmt
+   ```
+
+2. Run `swag init` to generate the documentation
+
+   ```bash
+   cd <project-root>
+   swag init --output api --generalInfo ./server/server.go --dir ./,./api/v1
+   ```
+
+> If the API gets a new version, you'll need to add the file system path to swag's `--dir` parameter.