update docs

[ranocha]

• Show versions in benchmarks
• Fix introduction rendering
• Mention periodic upwind operators in the introduction; closes Non-central periodic operators? #136

[codecov]

Codecov Report

Merging #137 (f405e06) into main (4e084b8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #137   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files          31       31           
  Lines        4896     4896           
=======================================
  Hits         4409     4409           
  Misses        487      487           

Flag	Coverage Δ
unittests	90.05% <ø> (ø)

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[coveralls]

Coverage remained the same at 90.053% when pulling f405e06 on hr/docs into 4e084b8 on main.