Document new color_mode cookie (#17982)

* Add new color_mode cookie

* Update content/github/site-policy/github-subprocessors-and-cookies.md

Co-authored-by: Lee Dohm <1038121+lee-dohm@users.noreply.github.com>

* Empty commit for CI

Co-authored-by: Lee Dohm <1038121+lee-dohm@users.noreply.github.com>
Co-authored-by: Shati Patel <42641846+shati-patel@users.noreply.github.com>
Co-authored-by: Martin Lopes <martin389@github.com>

Author: 4 people

content/github/site-policy/github-subprocessors-and-cookies.md

@@ -50,6 +50,7 @@ Since the number and names of cookies may change, the table below may be updated
 | Service Provider | Cookie Name | Description | Expiration* |
 |:---|:---|:---|:---|
 | GitHub | `app_manifest_token` | This cookie is used during the App Manifest flow to maintain the state of the flow during the redirect to fetch a user session. | five minutes |
+| GitHub | `color_mode` | This cookie is used to indicate the user selected theme preference. | session | 
 | GitHub | `_device_id` | This cookie is used to track recognized devices for security purposes. | one year |
 | GitHub | `dotcom_user` | This cookie is used to signal to us that the user is already logged in. | one year |
 | GitHub | `_gh_ent` | This cookie is used for temporary application and framework state between pages like what step the customer is on in a multiple step form. | two weeks |