Add rpk registry context reference pages (#588)

The single-sourced Schema Registry contexts page in the docs repo
links to rpk-registry-context-list and rpk-registry-context-delete.
Cloud-docs lacked wrapper pages for these commands, so the xrefs
broke when the content was included into cloud-docs.

Add thin wrapper pages following the existing
include::ROOT:reference:...[tag=single-source] pattern, plus the
corresponding nav entries between compatibility-level and mode.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: micheleRP

modules/ROOT/nav.adoc

@@ -605,6 +605,9 @@
 **** xref:reference:rpk/rpk-registry/rpk-registry-compatibility-level.adoc[]
 ***** xref:reference:rpk/rpk-registry/rpk-registry-compatibility-level-get.adoc[]
 ***** xref:reference:rpk/rpk-registry/rpk-registry-compatibility-level-set.adoc[]
+**** xref:reference:rpk/rpk-registry/rpk-registry-context.adoc[]
+***** xref:reference:rpk/rpk-registry/rpk-registry-context-list.adoc[]
+***** xref:reference:rpk/rpk-registry/rpk-registry-context-delete.adoc[]
 **** xref:reference:rpk/rpk-registry/rpk-registry-mode.adoc[]
 ***** xref:reference:rpk/rpk-registry/rpk-registry-mode-get.adoc[]
 ***** xref:reference:rpk/rpk-registry/rpk-registry-mode-reset.adoc[]

modules/reference/pages/rpk/rpk-registry/rpk-registry-context-delete.adoc

@@ -0,0 +1,3 @@
+= rpk registry context delete
+
+include::ROOT:reference:rpk/rpk-registry/rpk-registry-context-delete.adoc[tag=single-source]

modules/reference/pages/rpk/rpk-registry/rpk-registry-context-list.adoc

@@ -0,0 +1,3 @@
+= rpk registry context list
+
+include::ROOT:reference:rpk/rpk-registry/rpk-registry-context-list.adoc[tag=single-source]

modules/reference/pages/rpk/rpk-registry/rpk-registry-context.adoc

@@ -0,0 +1,3 @@
+= rpk registry context
+
+include::ROOT:reference:rpk/rpk-registry/rpk-registry-context.adoc[tag=single-source]