Support `exclude_docs`

Since 1.5 mkdocs supports the new option [`exclude_docs`](https://www.mkdocs.org/user-guide/configuration/#exclude_docs)

This option is not reflected now by the Parser and the nav contains items for the non-existing files.

Start by tracing the Parser's handling of MkDocs configuration and how it builds the nav. Verify the behavior with a configuration using exclude_docs, and consider the work done when excluded files no longer appear as navigation items.

Written by the indexing model from the issue text.