Based on the proposed process proposed by Akkana Peck. Thanks Akk!
For now, it's only in the meson build, which is fairly terrible to use
as soon as we do custom build rules. Here are the list of issues:

- meson does not allow building in subdir (issue 2320 on meson tracker).
  Sure I could make several subdirs with meson in them. But here the
  future goal would be to be able to generate docs for other
  introspected languages, and maybe also other output formats (epub or
  whatnot). For this, since these are basically the same commands which
  are used, the best practice would be to have loops generating one
  target per language/format combination, reusing code rather than ugly
  copy-pasting in subdirectories' meson files).
- custom_target() requires the output parameter to be the complete list
  of generated files. But we have more than a thousand of them. It's not
  practical. Maybe we could try to find a way to generate the list from
  the contents of the .def files which are already exhaustive and exact.
- Install also requires the output list to be complete.
- I temporarily have these docs not generated by default (because the
  gtk-doc option is already crazy slow as it is, making meson near
  unusable for development if it's enabled). If you want to generate the
  docs, the commands are as following (yeah I don't understand what the
  target names are for since meson does not actually create targets with
  these names, so we have to use fake output names instead):

> ninja devel-docs/g-ir-docs/Gimp-python-html
> ninja devel-docs/g-ir-docs/GimpUi-python-html