Fix build warnings/errors in docs

[oruebel]

Motivation

This PR fixes numerous minor build warnings in the docs due bad heading underlines, use of hard-coded links instead of external links, etc.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1462 (3311edf) into dev (985e3e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1462   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files          37       37           
  Lines        2733     2733           
  Branches      455      455           
=======================================
  Hits         2117     2117           
  Misses        535      535           
  Partials       81       81           

Impacted Files	Coverage Δ
src/pynwb/behavior.py	100.00% <ø> (ø)
src/pynwb/validate.py	0.00% <ø> (ø)

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