CompatHelper: bump compat for "Documenter" to "0.27" for package docs

[github-actions]

This pull request changes the compat entry for the Documenter package from ~0.26 to ~0.26, 0.27 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.

[coveralls]

Pull Request Test Coverage Report for Build 926958321

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.525%

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Totals
Change from base Build 888541031:	0.0%
Covered Lines:	1734
Relevant Lines:	1778

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 926958321

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 97.776%

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Totals
Change from base Build 888541031:	0.3%
Covered Lines:	1934
Relevant Lines:	1978

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💛 - Coveralls