Migrate OTEPs to the Specification repository

[carlosalberto]

Tracking issue for https://github.com/open-telemetry/oteps/pull/266 in order to fully complete the migration.

• [x] Migrate OTEPs along with their history (#4288)
• [x] Move issues from the OTEPs repository (to either the Specification or semantic conventions).
• [x] Clarification on the README & each OTEP that these are proposal of intent, not actual specification instructions (#4308).
• [ ] Normative language usage clarification.
• [x] Define how OTEP number are assigned.
• [ ] Archive the OTEPs directory along with a warning message.

[trask]

@carlosalberto can you clarify what's remaining on this? thanks!