docs: add an index and move the docstrings to a reference page

[digital-carver]

One disadvantage of using Autodocs now is that the nice, easy-to-navigate list of names in the left sidebar that's in the stable docs, is no longer there in the dev docs.

This PR uses Documenter's @index feature to generate an index of names as a replacement. I also moved the (@autodoc-ed) docstrings to a separate reference page, since it looks nicer to have just a quick list of functions at a glance in the homepage, instead of showing the intimidating full listing of docstrings right away.

[codecov-commenter]

Codecov Report

Merging #113 (cc473a5) into master (403cdca) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   90.77%   90.75%   -0.03%     
==========================================
  Files           1        1              
  Lines         401      400       -1     
==========================================
- Hits          364      363       -1     
  Misses         37       37              

see 1 file with indirect coverage changes

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