use zot as an extension name, ext as a component and search as a module

add endpoints field in ext discover api

distribution spec extension discover api has endpoints field required.

https://github.com/opencontainers/distribution-spec/blob/main/extensions/_oci.md#extensions-property-descriptions

Signed-off-by: Shivam Mishra <shimish2@cisco.com>

pkg/extensions/_zot.md

@@ -0,0 +1,12 @@
+`zot` [extension](#references)
+===
+
+`zot` extension has following components accessible under `/v2/_zot` endpoint.
+
+Component | Endpoint | Description
+--- | --- | ---
+[`search`](search/search.md) | `/v2/_zot/ext/search` | efficient and enhanced registry search capabilities using graphQL backend
+
+
+# References
+[1] https://github.com/opencontainers/distribution-spec/tree/main/extensions