Link to docs referenced from Driver.md

[kastiglione]

This change adds markdown links for references to other docs. Without this, the docs are bare text.

Because of wrapping, the diff is not clear. A --word-diff shows the following substitutions:

[-DriverInternals.rst-]{+[DriverInternals.rst](DriverInternals.rst)+}
[-DependencyAnalysis.rst-]{+[DependencyAnalysis.rst](DependencyAnalysis.rst)+}
[-Lexicon.rst-]{+[Lexicon.rst](Lexicon.rst)+}
[-DependencyAnalysis.rst.-]{+[DependencyAnalysis.rst](DependencyAnalysis.rst).+}

[jrose-apple]

Thanks, Dave!

[jrose-apple]

@swift-ci Please smoke test

[kastiglione]

@jrose-apple The docs are really well written and contain a lot of great information. Thank you to all who have taken the time to write them up.