[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>

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