Improve docs sidebar layout

[timhoffm]

This PR improves the layout of the sidebar:

• Move the search field to the top (should be easily accessible without the need to scroll).
• Reduce the heading font size so that "Table Of Contents" is not wrapped.

Here's a screenshot of the change (left: before, right: after):

[codecov]

Codecov Report

Merging #5568 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5568   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files         117      117           
  Lines       25713    25713           
  Branches     2493     2493           
=======================================
  Hits        24128    24128           
  Misses       1256     1256           
  Partials      329      329

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[curiousjazz77]

Nice improvement!

[nicoddemus]

Thanks! appreciate it! 👍