docs: Update list of officially supported browsers

[acdvorak]

• Remove "Opera". We don't test in it, and it uses the same rendering engine as Chrome (Blink)
• Reorganize list into a map of browsers to OSes (I find it easier to read)

[codecov-io]

Codecov Report

Merging #3082 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3082   +/-   ##
======================================
  Coverage    98.3%   98.3%           
======================================
  Files         101     101           
  Lines        4368    4368           
  Branches      564     564           
======================================
  Hits         4294    4294           
  Misses         74      74

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[williamernest]

LGTM!