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

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.

Related to T1330.