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-sub.md

@@ -0,0 +1,8 @@
+# gnmic sub
+
+> This command is an alias of `gnmic subscribe`.
+> More information: <https://gnmic.kmrd.dev/cmd/subscribe>.
+
+- View documentation for the original command:
+
+`tldr gnmic subscribe`