"scons install" broken since tools rewrite

The tools have been reimplemented in Go and now live in a new repository: https://github.com/mongodb/mongo-tools
The old C++ tools have been removed from mongodb/mongo.

Update the docs to reflect this change.

ORIGINAL DESCRIPTION:

When following the installation instructions in https://github.com/mongodb/mongo/blob/master/docs/building.md the `scons install` step fails with:

````
scons: *** [/xxx/mongodb/bin/bsondump] Source `src/mongo-tools/bsondump' not found, needed by target `/xxxx/mongodb/bin/bsondump'.
scons: building terminated because of errors.
```

Looking at the discussion in https://github.com/Homebrew/homebrew/pull/33652 it seems like it's need to separately compile the tools using Go. It's unclear what the correct installation method is.