list all doc dependencies in Project.toml

[jishnub]

These are used in docs/make.jl, and having these listed in docs/Project.toml makes the local documentation build succeed when the docs environment is loaded.

[codecov]

Codecov Report

Merging #767 (c435816) into master (373cbb6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #767   +/-   ##
=======================================
  Coverage   30.89%   30.89%           
=======================================
  Files          15       15           
  Lines         848      848           
=======================================
  Hits          262      262           
  Misses        586      586           

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