Generate a single-file api reference

Add a rule to generate a single-file html version of the
libflatpak api reference, for integration into readthedocs.

This is pretty plain, but serves the purpose.

Closes: #2526
Approved by: matthiasclasen
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f