Update documentation to notify about MacOS 11.0 availability

[maxim-lobanov]

Why:

Recently, GitHub Actions has added support of MacOS Big Sur (11.0) environment. See details in announcement: actions/runner-images#1814

What's being changed:

• Update the list of available environments
• Provide link to Big Sur documentation
• Add Note that MacOS 11.0 is available as a preview. The same note that we have for Ubuntu 20.04

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[jenschelkopf]

LGTM

[rachmari]

Thanks @maxim-lobanov for putting this together!

LGTM! 🚀 I'll get this merged right away.

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours.

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