Docs: Add guidelines for enabling keyboard navigation on macOS

What does this MR do?

Follow-up from #342841 (comment 733507485).

This MR adds steps for enabling full keyboard navigation on macOS, as well as additional resources for troubleshooting/enabling keyboard navigation on different browsers.

Full keyboard navigation seems to be enabled by default for google chrome, but not so for other browsers like Firefox and Safari on macOS. Since gitlab supports these browsers, this section should help us develop accessibility for these browsers.

More info on this issue can be found here: https://www.a11yproject.com/posts/2017-12-29-macos-browser-keyboard-navigation/

Related issues

#342841 (closed)

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