Travis CI -> GitHub workflow: doc8

[kiwitcms-bot]

Kiwi TCMS needs help migrating certain test jobs from Travis CI into GitHub workflows. In particular the doc8 job.

Definition of Done:

• "doc8" job has been removed from Travis CI
• "lint / doc8 (3.7)" job is enabled for GitHub workflows. See .github/workflows/lint.yml
• The new job reports its result on the pull request

This issue is part of Kiwi TCMS open source bounty program. For more information see the link(s) in bounty-program milestone

[Prome88]

Will each linter have it's own *.yml file or all should got to lint.yml as separate jobs?

From the commit msg for lint.yml:

Add .github/workflows/lint.yml, which lists the GitHub actions, which we
will use for linting. (We may have more linting actions in the same
file, for example pylint, or more actions is separate files)

[atodorov]

Will each linter have it's own *.yml file or all should got to lint.yml as separate jobs?

this one goes in lint.yml, see it's desired name in the description above. There are other jobs which would go elsewhere.