Summary: This adds doxygen which allows us to browse MD files and  C++ documentation.

Test Plan: Jenkins

Reviewers: #engineering, oazizi, michelle, philkuz

Reviewed By: #engineering, oazizi

Differential Revision: https://phab.corp.pixielabs.ai/D1029

GitOrigin-RevId: 573320b92ac49c4482ca5aa6dbc8438eb479f2ff