CI: Build wayfire API documentation using doxygen (#9)

[soreau]

This adds a CI job that generates documentation from wayfire docstrings using doxygen. To use it, follow these steps:

1. This PR must be merged to the default master branch for this to work, you cannot test it before this. To see a successful workflow of this pull request, please see https://github.com/soreau/wayfire/actions/runs/23016563839
2. Navigate to the Actions tab
3. Click Manual Wayfire Documentation Generation in the left pane
4. Click the green button labeled Run workflow with the dropdown
5. Select master branch
6. Click Run workflow
7. Check the CI output for the .zip archive artifact url
8. Download the artifact and extract the single html directory
9. Open the index.html file within the html directory in your browser