Delete all the magic around MkDocs YAML that processes the `mkdocs.yml`.

file and
define `readthedocs` theme automatically based on a feature flag,
`MKDOCS_THEME_RTD`.

This PR removes:

- automatically defining `readthedocs` as theme when `MKDOCS_THEME_RTD` feature
  flag is defined.
- processing `mkdocs.yml` to add internal JS and CSS (embed and analytics) files
  automatically

This is another step forward on removing all the magic we perform on behalf the
users and being more explicit about how to configure each doctool.

Reference:

* readthedocs/addons#72 (comment)

Closes #8529

Listen to `Project.post_save` signal and enable Addons if the project was
created after a specific date, it's MkDocs and it's the first time the
`AddonsConfig` is created.

Co-authored-by: Santos Gallegos <stsewd@proton.me>