Add some documentation for Helix mode (#35641)

Because there is literally no mention of it in the docs

Release Notes:

- N/A

---------

Co-authored-by: ponychicken <183302+ponychicken@users.noreply.github.com>
Co-authored-by: Ben Kunkle <ben@zed.dev>

docs/src/key-bindings.md

@@ -14,7 +14,7 @@ If you're used to a specific editor's defaults you can set a `base_keymap` in yo
 - TextMate
 - None (disables _all_ key bindings)
 
-You can also enable `vim_mode`, which adds vim bindings too.
+You can also enable `vim_mode` or `helix_mode`, which add modal bindings. For more information, see the documentation for [Vim mode](./vim.md) and [Helix mode](./helix.md).
 
 ## User keymaps
 
@@ -119,7 +119,7 @@ It's worth noting that attributes are only available on the node they are define
 
 Note: Before Zed v0.197.x, the ! operator only looked at one node at a time, and `>` meant "parent" not "ancestor". This meant that `!Editor` would match the context `Workspace > Pane > Editor`, because (confusingly) the Pane matches `!Editor`, and that `os=macos > Editor` did not match the context `Workspace > Pane > Editor` because of the intermediate `Pane` node.
 
-If you're using Vim mode, we have information on how [vim modes influence the context](./vim.md#contexts)
+If you're using Vim mode, we have information on how [vim modes influence the context](./vim.md#contexts). Helix mode is built on top of Vim mode and uses the same contexts.
 
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