gnmic: add page and subpages (#8367)

* gnmic - add

* Change file extension

* Fix newlines

* update gnmic pages and add gnmic sub alias

* fix link

* add period

* fix missing punctuation

* fix ip:port formatting

* Implemented @adrienthebo suggestions

* missing period

* @EmilySeville7cfg suggestions

* remove --insecure and write out sub

pages/common/gnmic-subscribe.md

@@ -0,0 +1,24 @@
+# gnmic subscribe
+
+> Subscribe to a gnmic network device state updates.
+> More information: <https://gnmic.kmrd.dev/cmd/subscribe>.
+
+- Subscribe to target state updates under the subtree of a specific path:
+
+`gnmic --address {{ip:port}} subscribe --path {{path}}`
+
+- Subscribe to a target with a sample interval of 30s (default is 10s):
+
+`gnmic -a {{ip:port}} subscribe --path {{path}} --sample-interval 30s`
+
+- Subscribe to a target with sample interval and updates only on change:
+
+`gnmic -a {{ip:port}} subscribe --path {{path}} --stream-mode on-change --heartbeat-interval 1m`
+
+- Subscribe to a target for only one update:
+
+`gnmic -a {{ip:port}} subscribe --path {{path}} --mode once`
+
+- Subscribe to a target and specify reponse encoding (json_ietf):
+
+`gnmic -a {{ip:port}} subscribe --path {{path}} --encoding json_ietf`