ci: test on alternative operating systems; ignore doc updates

[Fdawgs]

• Stops CI from running of documentation is updated (waste of a run)
• Tests on macOS and Windows alongside Ubuntu, to ensure fluent-json-schema runs on all operating systems (Windows is the big one to test, just due to how it handles forward/back slashes differently, and single quotation marks from unix)

Checklist

• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[coveralls]

Pull Request Test Coverage Report for Build 839090865

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 remained the same at 99.554%

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Totals
Change from base Build 834338069:	0.0%
Covered Lines:	408
Relevant Lines:	408

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💛 - Coveralls

[aboutlo]

LGTM

[mcollina]

lgtm