add documentation for the indexing statistics terms on the repository settings page

Created by: ggilmore

Each repository has its own settings page (example: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/settings/).

The "indexing" section of this settings page contains some statistics about the repository's index:

It would be great if there was some additional documentation that explained what each of these terms meant. I think this can happen in two stages:

• create a new page in our documentation that explains this page and the statistics on it
• stretch: add hover tool tips to the repository settings page. When a user hovers over a term, they can get a short explanation for what the term means.
  • I believe this can be accomplished by adding a data-tooltip attribute to the term's react component. See this link for how this is implemented on the extension registry page
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