Add :h[elp] vim command (#30179)

@jyn514 mentioned that this would be nice to have while trying out zed,
and it seemed simple enough so I added it.

Release Notes:

- Added `OpenDocs` action to open Zed's docs in a browser, aliased to
`:h[elp]` in vim.

crates/zed_actions/src/lib.rs

@@ -35,6 +35,7 @@ actions!(
         Quit,
         OpenKeymap,
         About,
+        OpenDocs,
         OpenLicenses,
         OpenTelemetryLog,
     ]