Tracking issue for open-telemetry/oteps#266 in order to fully complete the migration.

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