Support stability definitions from OTEP 232

[lmolkova]

See open-telemetry/opentelemetry-specification#4061

We need to support new formally defined (in OTEP 232) maturity levels:

• Development - add
• Alpha - add
• Beta - add
• Release Candidate - add
• Stable - no change
• Deprecated - no change
• Unmaintained - don't add for now

Experimental stability should default to development, but can be changed for each group/document to a higher (non-stable) level.

This would mean:

• updating the tooling and schema for the new levels
• updating all yaml and markdown files
• updating document statuses

[trask]

I think @jack-berg's open-telemetry/opentelemetry-specification#4061 (comment) could apply to this repo as well

Not sure how appropriate development is for spec language. Given that even experimental spec components require prototypes to merge, it seems like most new features in the spec come in at the alpha level of stability.