fix: handle missing pages in mkdocs nav section

[pawelsikora]

Add handling of missing (may be on purpose) navigation section (nav) in mkdocs.yml config file.

If pages are missing in nav, plugin will create the required pages from the filesystem structure, e.g.

parent1/parent2/.../your_page.md:

My Page

                              ...

In the following manner: Confluence Space:

• Main Parent:
  • Parent1:
    • Parent2: ...:
      • My Page

Fixes #2