Expand section "Contributing to Documentation"

[humitos]

We have this section https://docs.readthedocs.io/en/stable/contribute.html#contributing-to-documentation that doesn't have too much content and links to https://docs.readthedocs.io/en/stable/development/docs.html

However, the content of the second page is not great. We should explain how to make some documentation changes just by using the PR builder: editing a file using GitHub UI and previewing the changes at Read the Docs.

Besides, we could create a section that explains how to do it locally with Sphinx if the changes are going to be bigger: re-structure of documentation, whole new page, etc

Ref: #7994 (comment)