[SE-5509] docs: periodic builds discovery
Description
This MR adds a discovery for Periodic Builds using Grove (and OpenFAAS).
Resolves #44 (closed)
Testing instructions
- checkout the branch
- setup virtualenv
- run
mkdocs serve
- navigate to http://localhost:8000/discoveries/0005-periodic-builds/
- proofread the changes
Edited by Boros Gábor