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

Junseong Park created 

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

Release Notes:

- N/A

Change summary

assets/settings/default.json |  8 +++--
docs/src/configuring-zed.md  | 56 ++++++++++++++++++++++++++++++++++++++
2 files changed, 61 insertions(+), 3 deletions(-)

Detailed changes

assets/settings/default.json 🔗 

@@ -15,9 +15,11 @@
   // text editor:
   //
   // 1. "VSCode"
-  // 2. "JetBrains"
-  // 3. "SublimeText"
-  // 4. "Atom"
+  // 2. "Atom"
+  // 3. "JetBrains"
+  // 4. "None"
+  // 5. "SublimeText"
+  // 6. "TextMate"
   "base_keymap": "VSCode",
   // Features that can be globally enabled or disabled
   "features": {

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.