CompatHelper: bump compat for Documenter to 1 for package docs, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Documenter package from 0.26, 0.27 to 0.26, 0.27, 1 for package docs.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.08%. Comparing base (a1010e4) to head (cf45ea3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1778   +/-   ##
=======================================
  Coverage   86.08%   86.08%           
=======================================
  Files         144      144           
  Lines        8696     8696           
=======================================
  Hits         7486     7486           
  Misses       1210     1210           

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[penelopeysm]

(@devmotion, as you're the most active committer!) would it be possible to merge this please? Documenter v1.3 would generate a nice objects.inv file which lets us link to Distributions docs easily 🙂

[penelopeysm]

I checked it locally and this PR would probably also need a warnonly=true keyword argument to be added to makedocs(), as Documenter 1.x upgrades the warnings to errors by default.