New theme for MkDocs.

Features:

 - Fully responsive (PC, Tablet & Mobile).
 - Quite good UX (auto-scrolling TOC, main navigation, previous & next etc.)
 - Previous & Next buttons with articles' titles.
 - HQ Docker logo
 - Lighter footprint
 - Faster browser rendering
 - Mobile CSS (embedded in a single file).

Improvements & fixes:

 - Fix issues of scrolling TOC and main nav.
 - Fixed issues of search on mobile
 - Fixed issues of display of prev & next
 - TOC table's view on mobile & tablets
 - Fonts
 - Fonts sizes
 - Increased readability
 - Custom dropdowns (mobile compliant)

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5150 from ostezer/mkdocs-theme-3

New theme for MkDocs.