Overhaul and cleanup the repository

[bwplotka]

Let's double check what's still needed this repository and what's important to improve

• Can we remove legacy, is it helpful to keep this? It's all in git history anyway.
• Write docs on how to develop and use src code e.g. should every spec change tests?
• Move the CI to GH Actions.
• Update or remove Timeline https://github.com/prometheus/OpenMetrics/blob/main/TIMELINE.MD
• Update maintainers.
• Fix python tests after move (e.g. failing here)

cc @RichiH

[bwplotka]

Should we simply archive given we are moving all artifacts across docs (spec), Prometheus repo (scraping), compliance (tests).

Website is still to move, but we could play with DNS to point to spec and Prometheus website.

[bwplotka]

For website I propose we:

• Set DNS for openmetrics.io to same as prometheus.io (added ticket to CNCF service desk)

• https://prometheus.io/docs/specs/om/ will by redirecting to the latest stable OpenMetrics (currently 1.0). See Redirect top level spec/<proj> dir to the latest version docs#2773 and Redirect openmetrics.io domain to om dir docs#2774