[chore] Change default database in example config to sqlite, update docs. (#4174)

As discussed in Matrix chat.

* Change DB in example config to sqlite.
* Clarify config settings which only apply to postgres.
* Update getting started docs; no need to change example config any more.
* In "single board computers" mention SD card corruption risk.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [X] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [X] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [X] I/we have not leveraged AI to create the proposed changes.
- [X] I/we have performed a self-review of added code.
- [X] I/we have made any necessary changes to documentation.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4174
Co-authored-by: Phil Hagelberg <phil@hagelb.org>
Co-committed-by: Phil Hagelberg <phil@hagelb.org>

example/config.yaml

@@ -141,8 +141,8 @@ trusted-proxies:
 
 # String. Database type.
 # Options: ["postgres","sqlite"]
-# Default: "postgres"
-db-type: "postgres"
+# Default: "sqlite"
+db-type: "sqlite"
 
 # String. Database address or parameters.
 #
@@ -157,29 +157,29 @@ db-type: "postgres"
 #
 # Examples: ["localhost","my.db.host","127.0.0.1","192.111.39.110",":memory:", "sqlite.db"]
 # Default: ""
-db-address: ""
+db-address: "sqlite.db"
 
-# Int. Port for database connection.
+# Int. Port for postgres database connection; ignored for sqlite.
 # Examples: [5432, 1234, 6969]
 # Default: 5432
 db-port: 5432
 
-# String. Username for the database connection.
+# String. Username for postgres database connection.
 # Examples: ["mydbuser","postgres","gotosocial"]
 # Default: ""
 db-user: ""
 
-# String. Password to use for the database connection
+# String. Password to use for postgres database connection.
 # Examples: ["password123","verysafepassword","postgres"]
 # Default: ""
 db-password: ""
 
-# String. Name of the database to use within the provided database type.
+# String. Name of the database to use for postgres database connection.
 # Examples: ["mydb","postgres","gotosocial"]
 # Default: "gotosocial"
 db-database: "gotosocial"
 
-# String. Disable, enable, or require SSL/TLS connection to the database.
+# String. Disable, enable, or require SSL/TLS connection to postgres.
 # If "disable" then no TLS connection will be attempted.
 # If "enable" then TLS will be tried, but the database certificate won't be checked (for self-signed certs).
 # If "require" then TLS will be required to make a connection, and a valid certificate must be presented.