Docs: review and update the whole FAQ page. (#10732)
* Docs: remove old question from FAQ
We are not failing builds that don't ouput an `index.html` on the root.
Due to that, this question is not valid anymore.
* guilabel
* Update question about `sphinx.` and `mkdocs.` configuration
* Remove feature flags question
We removed 99% of them and we don't want to expose them anymore.
* RTD -> Read the Docs
* URL update
* Fusion two theme questions
* Update image scaling question
* Update install a Python package question
* Update reference
* We are not using always-eager anymore
These dependencies are not upgraded automatically.
* Remove question about Anaconda project YAML
This is a pretty old project that's not used anymore.
* Sphinx issues fixed
