* set up Doxygen
- `.gitignore`: ignore Doxygen output
- limit Doxygen search to relevant paths
- move extra `.md`s to docs/
- `Makefile`: add Doxygen target
- use as much SVG as possible for Doxygen diagrams
- alias @meme as an \xrefitem
* new github workflow: run doxygen and push to gh pages
Erin Moon