Add ezgl Doxygen api documentation to to the VTR docs

[vaughnbetz]

Proposed Behaviour

EZGL's api should be documented under the vtr documentation.

Current Behaviour

EZGL is documented with doxygen, but we aren't building or linking its documentation into the VTR read the docs pages.

Possible Solution

Build the ezgl doxygen html docs as part of the documentation build. Put a new EZGL tab under vtrutil api.

There is a tutorial on how to add new tabs to our docs at https://docs.verilogtorouting.org/en/latest/dev/code_documentation/#validation-of-doxygen-comments-updating-api-reference
(it is probably not as easy to follow as it could be -- you can update it if you determine it is hard to follow!).
We build the documentation on each pull request, so the easiest way to check if the documentation built properly is just to make a PR and check (you can click on the documentation test link in the PR). That should also be added to the tutorial / doxygen guide; you could do that as part of this task.