doc: improve scrolling, various CSS tweaks

- Made scrolling of the sidebar not affect the main page by using a combination
  of overflow:hidden and overflow:scroll.
- Changed the scrollbar of the sidebar to be visible again for accessibilty
  reasons and removed the indication gradient because it was causing too much
  issues.
- In WebKit-based browsers, the scrollbar received custom styling making it
  appear to be outside of the sidebar.
- The main content is no longer limited to 702px width, but now uses all
  available space.
- Changed the background of the code blocks to a very similar, but neutral
  color and made inline blocks the same color.
- Made inline code blocks inside italic sections not italic.
- Simplified styling of api_stability classes by introducing a common class
  name for the stability levels.
- Fixed various issues related to the green hover background on links.
- Fixed code box overflow outside the main content area.
- Various minor tweaks to paddings and margins.
- Cleaned up numbers in the stylesheet, removing unnecessary units, decimals
  and empty selectors.

PR-URL: #5198
Reviewed-By: James M Snell <[email protected]>