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.md

@@ -0,0 +1,25 @@
+# gnmic
+
+> A gNMI command-line client.
+> Manage gNMI network device configuration and view operational data.
+> More information: <https://gnmic.kmrd.dev>.
+
+- Request device capabilities:
+
+`gnmic --address {{ip:port}} capabilities`
+
+- Provide a username and password to fetch device capabilities:
+
+`gnmic --address {{ip:port}} --username {{username}} --password {{password}} capabilities`
+
+- Get a snapshot of the device state at a specific path:
+
+`gnmic -a {{ip:port}} get --path {{path}}`
+
+- Update device state at a specific path:
+
+`gnmic -a {{ip:port}} set --update-path {{path}} --update-value {{value}}`
+
+- Subscribe to target state updates under the subtree at a specific path:
+
+`gnmic -a {{ip:port}} subscribe --path {{path}}`