make-doc: refactor doc build system to build the doc as a whole

• back to assets only building in each package
• run sphinx-autodoc on the whole swh package at once
• add a tox.ini file with 2 sphinx environments:
  • sphinx: build the doc reading sources from installed swh subpackages,
  • sphinx-dev: build the doc from source swh subpackages in ..

we do not really need a local standalone buildable sphinx project in each swh subpackage, so running a full 'make html' in each of them is not required any more.

Since we want to build the doc at once instead of building each swh subpackage independantly, we need to run apidoc on the whole swh package.

This is done by actually running the apidoc on the pip installed swh packages instead of the source repositories, since sphinx-apidoc does not seem to support multiple MODULE_PATH as cmdline argument.

So it's best to use tox to build the doc.

Also note that this evolution of the doc build system requires modifications in every swh-*/docs/index.rst file to fix the module indexes.

Related to #1330.

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Migrated from D696 (view on Phabricator)