Prepare code for custo mkdocs.yaml location

[stsewd]

This fix #3249

Our docs mention that we search for the mkdocs.yaml inside the docs directory, but that's a lie, we only search in the root of the project. Also, this will prepare our code for the mkdocs key of the new yaml file.

[stsewd]

I need to do this for the others builders and use the --config-file option and a manual QA

[stsewd]

Some changes depend on #4041

[stsewd]

OK, I tested this manually with two projects (mkdocs and #3249 (comment)) it works as expected, going to block this due #4041

[stsewd]

Updated and tested, just to note that this feature doesn't work as the initially reported issue because of https://www.mkdocs.org/about/release-notes/#stricter-directory-validation. this is more useful for people that have a layout like:

• someproject/docs/
• someproject/mkdocs.yml
• readme.md

So, people need to install a previous version of mkdocs for it to works in a structure like the above.

For that reason, I decided to no implement that.

Is this PR still useful?

Yes, with this we can support the mkdocs.configuration key. We can modify this later to read the value from the rtd configuration file. Also, I had added some tests :)

[ericholscher]

Looks good. 👍