Switch to checking for dead links in the docs/CI HTML via Lychee

[sbryngelson]

This is an attempt to try to fix #671 and #716 at the same time

This PR moves the link checking to the HTML files created by the documentation build/CI.

Lychee is now used to check for dead links.

It works, and finds dead links that have been around a long time. It is currently configured to pass CI even if it finds dead links, but once the dead links are fixed, we will change this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.96%. Comparing base (e0317d7) to head (c1c1ce5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #733   +/-   ##
=======================================
  Coverage   42.96%   42.96%           
=======================================
  Files          61       61           
  Lines       16339    16339           
  Branches     1881     1881           
=======================================
  Hits         7020     7020           
  Misses       8278     8278           
  Partials     1041     1041           

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[sbryngelson]

this isn't right. need to switch to something that checks HTML not markdown. trying what I
discuss here #671

update: fixed.