Add action to generate documentation

[schornakj]

Adds a CI action that runs when the devel branch is pushed to. The action generates HTML documentation from the project readme and headers and pushes it to this repo's gh-pages branch.

[schornakj]

I ran this action on a test branch on my own fork of this repo to generate a preview of the docs: https://schornakj.github.io/yak-1/doxygen/html/index.html

Couple of things to fix down the road:

• Images in the readme don't appear in the webpage
• It would be nice to generate a temporary test version of the docs in PRs. As currently configured the docs are only updated when the devel branch updates, i.e. when PRs are merged in.