docs: add missing index pages

[leohhhn]

Description

This PR adds reformats "Overview" pages into section index pages. Important changes are in sidebars.js.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.03%. Comparing base (831bb6f) to head (739dd9e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
- Coverage   47.74%   45.03%   -2.71%     
==========================================
  Files         393      464      +71     
  Lines       61629    67975    +6346     
==========================================
+ Hits        29424    30615    +1191     
- Misses      29734    34785    +5051     
- Partials     2471     2575     +104     

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[gfanton]

lgtm