Remove duplication of digraph encoding information in the docs

[William-Kenyon]

Progress on #861, getting rid of the duplicated information about digraph encoding across DigraphFile, ReadDigraphs, and WriteDigraphs, by having them point to Encoding Formats. Encoding Formats was also missing file extensions and the pickling and plain text encodings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (c4ced48) to head (996cbf4).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
+ Coverage   97.46%   97.47%   +0.01%     
==========================================
  Files          51       51              
  Lines       21191    21301     +110     
  Branches      639      639              
==========================================
+ Hits        20653    20763     +110     
  Misses        473      473              
  Partials       65       65              

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[mtorpey]

This might sound fussy, but:

Could you please redo this with meaningful changes separated from whitespace changes? Currently it's quite hard to review.

Two separate commits would make this possible.

[james-d-mitchell]

Thanks @William-Kenyon, this is a good set of changes to make. Please adjust according to my comments, and then I'll be happy to merge.

[mtorpey]

I believe all James's comments have been resolved. Thanks for the good work, @William-Kenyon!

Done