Migrate OTEPs to the Specification repository

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.

open-telemetry / opentelemetry-specification