update documentation generation guidelines

[adit-0132]

fixes #2518

This PR updates the local documentation build instructions to match the current
repository layout.

The docs previously described a Makefile-based workflow (make html,
make livehtml, make cleandocs), but the repository does not include a
Makefile and the docs are built by invoking Sphinx directly.

The instructions are updated to use sphinx-build and sphinx-autobuild,
while preserving the existing structure and intent of the guide.

[OriolAbril]

@adit-0132 thanks for reporting, we'll open a PR to fix this very soon. The recommended way is to use tox instead of using sphinx directly.

You'll probably have seen this in the PR template already but note that most work on ArviZ features will not happen in this repository but instead on arviz-base, arviz-stats or arviz-plots. We are also working on contributing docs for them, but arviz-plots already has good pages on building docs and running tests

[adit-0132]

@OriolAbril got it, thanks for letting me know.

[OriolAbril]

#2522 updates the PR step by step tutorial. Here is the doc preview from that PR: https://arviz--2522.org.readthedocs.build/en/2522/contributing/pr_tutorial.html. Don't hesitate to comment on the PR if there are still things that aren't clear or don't work

Note: the tox list -m dev won't work on main yet, it needs the PR to be merged, as there is currently some metadata missing in our tox.ini. Other than that everything (in that page) should be up to date already. I am going over the rest as time allows.

I will close the PR but again, thanks for reporting the issues and opening the PR even if we eventually did not use these changes.