[BD-21] Add support for warnings in the featuretoggles Sphinx extension

[regisb]

Description: Add support for toggle_warning flags in the featuretoggles Sphinx extension

JIRA: https://openedx.atlassian.net/wiki/spaces/COMM/pages/1596358943

Dependencies: None. But useful for documenting edx-platform feature toggles.

Reviewers:

• @robrap

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• [ ] Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPi after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[openedx-webhooks]

Thanks for the pull request, @regisb! I've created BLENDED-560 to keep track of it in Jira. More details are on the BD-21 project page.

When this pull request is ready, tag your edX technical lead.

[robrap]

@regisb: Thanks. FYI: For the future, this should have been a minor version update. See https://open-edx-proposals.readthedocs.io/en/latest/oep-0047-bp-semantic-versioning.html Thanks.