API docs tarball for GIMP 2.99/3.0

Merged Jehan requested to merge wip/Jehan/docs-tarball into master

This will be useful for people who want to download the API docs without building GIMP fully themselves. Also it will be used by the developer website.

It used to be included in the source tarball (up to 2.10), but for now, I guess a dedicated tarball might be a better idea, especially as it's not source but built data.

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