Add doxygen job and GH pages

[Ryanf55]

Purpose

Create and host API documentation for grid_map_geo. Since this library is largely intended to be used in other projects, it is important to have a well documented C++ API.

Luckliy, we can use Doxygen to generate the docs, and view them on github pages with a few actions.

Demo

https://ryanf55.github.io/grid_map_geo/index.html

Reference

https://github.com/marketplace/actions/doxygen-action

[Ryanf55]

In order for the build to pass, the repo admin must enable Github pages. https://github.com/ethz-asl/grid_map_geo/actions/runs/7552085852/job/20560362797?pr=49#step:5:162

See this screenshot for the appropriate settings:

If you don't want to do the pages stuff quite yet, but want the docs, I can split this PR in two.

[Jaeyoung-Lim]

Merging to see if it works :)

[Ryanf55]

Merging to see if it works :)

Sort of? https://ethz-asl.github.io/grid_map_geo/

Hahah, it's the README, but not API docs. I'll look at it tonight.