Add documentation of GH actions and modernize them

Simplest use case is:
- on push to main affecting the source-file/md-file, render/push automatically

I tried making something from scratch to do this on /faq/ and gave up, not knowing there was already a working example of this here:

https://github.com/decentralized-identity/presentation-exchange/blob/master/.github/workflows/render-specs.yml

that I found out about here:

https://github.com/decentralized-identity/didcomm-messaging/issues/196