Publish two schemas versions with each release: stable and everything

[lmolkova]

When reporting schema version instrumentations should be able to differentiate which one they report - the stable or stable with experimental features.

For example, HTTP client span with schema URL pointing to v1.28.0 would have additional attributes if experimental ones are enabled.

To reduce the ambiguity, we can publish two schema urls:

• one that represents a stable part of semantic conventions, e.g. https://opentelemetry.io/schemas/1.29.0
• another one that contains all conventions, e.g. https://opentelemetry.io/schemas/1.29.0-alpha following semver pre-release version

This also allows to communicate stability guarantees to the consumers of this telemetry.

To support it, we'd need

• start publishing two documents on https://opentelemetry.io/schemas/* (existing schema-next files should be used for experimental part, we'll need new ones for stable part - which should be empty).
• this likely means we need to Automate schema-next generation #1426 first
• coordinate any code-generation and instrumentation changes with SIGs (should be minor)