docs: Add missing base_keymap option in configuring-zed.md (#18190)

Added `base_keymap`, an option that works in the editor but is missing
from the documentation.

Release Notes:

- N/A

docs/src/configuring-zed.md

@@ -93,6 +93,62 @@ Extensions that provide language servers may also provide default settings for t
 
 `boolean` values
 
+## Base Keymap
+
+- Description: Base key bindings scheme. Base keymaps can be overridden with user keymaps.
+- Setting: `base_keymap`
+- Default: `VSCode`
+
+**Options**
+
+1. VSCode
+
+```json
+{
+  "base_keymap": "VSCode"
+}
+```
+
+2. Atom
+
+```json
+{
+  "base_keymap": "Atom"
+}
+```
+
+3. JetBrains
+
+```json
+{
+  "base_keymap": "JetBrains"
+}
+```
+
+4. None
+
+```json
+{
+  "base_keymap": "None"
+}
+```
+
+5. SublimeText
+
+```json
+{
+  "base_keymap": "SublimeText"
+}
+```
+
+6. TextMate
+
+```json
+{
+  "base_keymap": "TextMate"
+}
+```
+
 ## Buffer Font Family
 
 - Description: The name of a font to use for rendering text in the editor.