Adds ability to search for documentation in repository root and in "./docs/"

[dduportal]

This PR introduce the ability to search, build and manage documentation when it's located in the ./docs sub directory.

Please note that it fixes an issue coming from #2 (and currently on master): if docs.Dockerfile is found into ./docs, then the build fails if requirements.txt is not next to it. But if we let mkdocs.yml at the root of the repo (on the same version), it also requires requirements.txt next to it.

[ldez]

👀

Some code can be simplify.

[dduportal]

• First batch of review applied. WDYT @ldez : did I forget something of what we discussed?
• This PR should be blocked: there is a regression for generating the menu, I'm working on this.

[dduportal]

Broken menu fixed, it was easy:

• Added a method to get the value of the docs_dir attribute from mkdocs.yml and returns the full path (parent dir of mkdocs.yml + found value of docs_dirs).
• Edited the menu.go to adapt the copy of the JS (and eventual custom CSS) for menu.
• And that's all: we don't have to rollback anything in Documentation Revamp traefik#4475 . This fix does not enforce anything about existing resources: structor adapts itself.

=> Validated on github.com/dduportal/traefik.

[ldez]

LGTM 👍

[dduportal]

OK, latest code works very well (on my fork :) ) for all of these scenarios:

• Build with current master (all branches have mkdocs.yml on the root). Even v1.1 works. Menu works for all.
• Build with master having mkdocs.yml in ./docs/. Even v1.1 works. Menu works for all.
• Build with a branch v15.0 tagged with v15.0.0 and v15.0.1. Even v1.1 works. Menu works for all. v1.7 is seen as v.1.6 today.