Improve documentation for path behavior in MrDocs

[fpelliccioni]

Based on the discussion in issue #529 and Alan's comment here, we need to improve the documentation to clarify that:

• Paths specified in the command line override the values in mrdocs.yml.
• All paths, whether in mrdocs.yml or the command line, are relative to the location of mrdocs.yml.
• The rationale is that mrdocs.yml is at a stable location in the project, and having paths relative to it avoids verbosity and inconsistency.

This will help users understand the expected behavior and avoid confusion.

[alandefreitas]

Fixed by https://github.com/cppalliance/mrdocs/pull/622