Add telemetry schemas to the specification

This merges the remaining bits of OTEP0152 to the specification
and links to already existing schema-related sections in the API.

The documents are marked as "Experimental", however I would like
to promote file_format_v1.0.0.md to Stable as soon as possible since we
already have published such files.

Related to open-telemetry/oteps#152

Author: tigrannajaryan

CHANGELOG.md

@@ -41,6 +41,10 @@ release.
 
 ### SDK Configuration
 
+### Telemetry Schemas
+
+Added telemetry schemas documents to the specification ([#2008](https://github.com/open-telemetry/opentelemetry-specification/pull/2008))
+
 ## v1.7.0 (2021-09-30)
 
 ### Context