Allow docs to be in the root directory

[oprypin]

• Closes Allow docs to be in the root directory mkdocs/mkdocs#3519
• Closes Support docs from the root directory. mkdocs/mkdocs#3450

If the config file is inside docs_dir, you get this message:

ERROR   -  Config value 'docs_dir': The 'mkdocs.yml' config file should not be inside the docs_dir.
           To allow this arrangement, please exclude the file (and other files as applicable) by adding the following configuration:

           exclude_docs: |
             /mkdocs.yml

If the site_dir is inside docs_dir, you get this message:

ERROR   -  Config value 'site_dir': The site_dir should not be inside the docs_dir as this leads to the build directory being copied into itself.
           To allow this arrangement, please exclude the directory (and other files as applicable) by adding the following configuration:

           exclude_docs: |
             /site
```Fix bypassing it when:

* a non-absolute config_file_path is passed on the command line
* the directory is not an *exact* parent of the other one, but is instead a "grandparent".

Also validate it in `mkdocs serve` because why reserve the check only to `mkdocs build`.