MkDocs theme improvements & fixes, including but not limited to:

 - Improved responsiveness
 - Padding, spacing, rendering and display adjustment fixes.
 - Removed somed unnecessary codes.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5188 from ostezer/mkdocs-mobile-nav-padding-fix

MkDocs theme improvements & fixes.