Adds documentation for the `DEFAULT_TITLE_SOURCE` setting.
- Added `DEFAULT_TITLE_SOURCE` to the config cheat sheet under `[repository.pull-request]`
- Added a "Default pull request title" section to the pull request usage docs explaining both modes (`first-commit` and `auto`)
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Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/docs/pulls/391
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: 0xGREG <gitea@freespeech.work>
Co-committed-by: 0xGREG <gitea@freespeech.work>