add type hints to `Medium` class and other improvements and fixes to docs

[oskooi]

Fixes #2539.

Various improvements and fixes to the docs including:

• add type hints for the Medium class
• mention version requirement for autograd dependency for adjoint solver
• fix or remove several broken links in one of the FAQs
• use consistent formatting for the type declarations
• use # as the anchor link (introduced in #2550)

[codecov-commenter]

Codecov Report

Merging #2554 (1d959bb) into master (37516cf) will decrease coverage by 0.42%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2554      +/-   ##
==========================================
- Coverage   73.91%   73.49%   -0.42%     
==========================================
  Files          18       18              
  Lines        5294     5294              
==========================================
- Hits         3913     3891      -22     
- Misses       1381     1403      +22     

Impacted Files	Coverage Δ
python/simulation.py	77.06% <ø> (ø)
python/geom.py	90.37% <100.00%> (-3.79%)	:arrow_down: