Add a link to documentation explaining timing buckets

[AdamTReineke]

It's hard for users to know what exactly is covered by the navigation timings. Let's add a link at the end of that panel to a new Markdown file in the docs folder that describes each bucket.

If you're grabbing this as a first issue, look at table in https://github.com/Microsoft/webperftoolbar/blob/develop/src/panels/navigation-timing.ts#L108 to find the defined times and how they're calculated (linked to an example line). Then create a navigation-timing.md file in the docs folder. Finally, add a link to that markdown file in the panel itself.