Improve search function

[ronso0]

there are a few aspects of the manual search that can be improved:

• include headers:
  it seems headers are ignored, for example "hidden tracks" should show The Mixxx Library > Hidden Tracks as top-most result
  https://manual.mixxx.org/2.3/en/search.html?q=%22Hidden+Tracks%22&check_keywords=yes&area=default#
  https://manual.mixxx.org/2.3/en/chapters/library.html#hidden-tracks

• show matches in hardware documentation last?

• show matches in Changelog last?