Add contexts and sync Schema Registry API spec to source (#65)

* Add contexts and sync Schema Registry API spec to source

* Fix invalid oneOf in schema_metadata_request for Swagger 2.0 compliance

Replace oneOf (string/number/boolean) with type: string in
additionalProperties of schema_metadata_request. The oneOf keyword
is not part of the Swagger 2.0 spec and breaks SDK generators.
Values are converted to strings by the server, so type: string
accurately reflects the API behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix style issues from docs review

- Fix version phrasing in overlay: "v26.1 and later" -> "version 26.1 or later"
- Fix comma splice in schema_metadata_request description
- Add missing description to defaultToGlobal on GET /config/{subject}
- Clarify "subject-version" -> "subject-version pairs" in summary
- Make POST /security/acls description add value beyond the summary

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Apply suggestion from @pgellert

Co-authored-by: Gellért Peresztegi-Nagy <pereszteginagy.gellert@gmail.com>

* Address comments from doc review

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Gellért Peresztegi-Nagy <pereszteginagy.gellert@gmail.com>

Author: 3 people

schema-registry/overlays/add-external-docs.yaml

@@ -15,4 +15,5 @@ actions:
 
         - [Use Redpanda Cloud with the Schema Registry API](https://docs.redpanda.com/redpanda-cloud/manage/schema-reg/schema-reg-api/)
         - [Use Redpanda Self-Managed with the Schema Registry API](https://docs.redpanda.com/current/manage/schema-reg/schema-reg-api/)
-        
+        
+        Schema Registry contexts (available in Redpanda version 26.1 or later) provide an isolated namespace for schemas, subjects, and configuration. You can use a context-qualified subject anywhere the API accepts a subject. In Redpanda Cloud, contexts are configurable only in BYOC and Dedicated clusters. See the [Redpanda Cloud](https://docs.redpanda.com/redpanda-cloud/manage/schema-reg/schema-reg-contexts/) or [Redpanda Self-Managed](https://docs.redpanda.com/current/manage/schema-reg/schema-reg-contexts/) documentation for details.