feat(theme-docs): allow customising default branch

[danielroe]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Not everyone will be preparing PRs against dev. It may be directly against main or against another branch. This PR allows customising this in settings (or automatically fetching the branch from GitHub). It defaults to dev, although this will be a breaking change in the (unlikely) case that users prefer a diff against dev even though they actually have a different default branch.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[danielroe]

Note this still hard codes PRs against docs/content prefix, which we should be able to detect and auto-update at build time (though I think that is for another day!)

[benjamincanac]

Thanks @danielroe!