Add the FastSS and Levenshtein modules to docs

[piskvorky]

I tried to find our new levenshtein module via google, and failed.

Upon investigation it turns out it's not documented at all (not API pages), so google doesn't even know about.

This PR adds the two missing module doc files, plus adds them to the top-level API ref listing.

[piskvorky]

@mpenkov could you please check what's up with the failing "Build wheels" test?

EDIT: n/m, looks like restart helped.

[codecov]

Codecov Report

Merging #3279 (752e477) into develop (7d7bb84) will increase coverage by 0.32%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #3279      +/-   ##
===========================================
+ Coverage    79.03%   79.35%   +0.32%     
===========================================
  Files           68       68              
  Lines        11781    11781              
===========================================
+ Hits          9311     9349      +38     
+ Misses        2470     2432      -38     

Impacted Files	Coverage Δ
gensim/similarities/levenshtein.py	91.66% <ø> (ø)
gensim/corpora/wikicorpus.py	93.75% <0.00%> (+6.25%)	⬆️
gensim/scripts/segment_wiki.py	92.37% <0.00%> (+21.18%)	⬆️

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[mpenkov]

Looks good, merging.