Please add microsoft/referencesource to the source indexer

I opened this issue previously on [microsoft/referencesource](https://github.com/microsoft/referencesource/issues/242), but then I realized that that repo appears to be abandoned, so I'm opening it here (sorry if this is seen as spam).

In the past, the website https://referencesource.microsoft.com/ pointed to a nice Source Browser webpage, powered by Kirill Osenkov's source browser, which worked just like the ones from https://source.dot.net/ and https://sourceroslyn.io/

Eventually, that webpage was taken down, and https://referencesource.microsoft.com/ is now just redirecting to the [microsoft/referencesource](https://github.com/microsoft/referencesource/) repo. And of course, lots of issues were opened, requesting the Source Browser to be brought back.

Now, the reason, I'm opening this issue, is because apparently, around last week, the Source Browser project was revived, and https://source.dot.net/ now has a new face, with a list of repositories that can be browsed:

I also noticed that this list appears to be growing: the last time I looked, there weren't so many repositories in that list.

So my request is: instead of bringing back the source browser at https://referencesource.microsoft.com/, just add [microsoft/referencesource](https://github.com/microsoft/referencesource/) to the list of repositories browsable at https://source.dot.net/

## Considerations

One side effect of adding that repository to https://source.dot.net/, is that every search for a class in the core library, or other framework libraries, would yield duplicate results: one from [dotnet/runtime](https://github.com/dotnet/runtime) repository, and one from [microsoft/referencesource](https://github.com/microsoft/referencesource/).

If this is a problem, then one possible solution, could be to make the search bar not search [microsoft/referencesource](https://github.com/microsoft/referencesource/) by default, unless it is explicitly selected in the drop-down list.

Start by locating the configuration or entry point that defines the repositories browsable at source.dot.net; the issue does not name a file or test. Add microsoft/referencesource to that repository list, account for its possible duplicate search results, and verify that it appears in the site's repository selector and can be browsed.

Written by the indexing model from the issue text.