[WIP] Define semantic convention group and mixed stability rules

[lmolkova]

Fixes #906.

Related: #1423

Key points:

• Allow stable groups with experimental attributes (as opt-in)
• Document some rules around it

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[jsuereth]

Subject to more specific rules, e.g. metric groups where you CANNOT change requirement_level without it being a breaking change.

[lmolkova]

this is allow-list for experimental groups - breaking, but allowed

[lmolkova]

I initially called it out in the stable group list below, but then decided that stable groups + stable attributes guarantees would belong in the https://github.com/open-telemetry/opentelemetry-specification/blob/0a78571045ca1dca48621c9648ec3c832c3c541c/specification/versioning-and-stability.md?plain=1#L217. WDYT?