Read the config from properdocs.yml and fall back to mkdocs.yml

[oprypin]

Support 'mkdocs.yml' as a fallback after 'properdocs.yml' is not found

Add a corresponding warning at INFO level

The order of preference can be seen directly in the code:

paths_to_try = ['properdocs.yml', 'properdocs.yaml', 'mkdocs.yml', 'mkdocs.yaml']

If the file is mkdocs.y*ml, then this warning is shown:

INFO    -  The configuration file 'mkdocs.yml' should be renamed to 'properdocs.yml', OR it should be passed explicitly on the command line: `-f mkdocs.yml`.
           Support for using this legacy file name as a fallback will eventually be removed from ProperDocs.