docs: remove the User spend page from AI Gateway docs (#29844)

## Description

Remove the **User spend** page from the AI Gateway and spend management
docs, since the page is hidden from the sidebar and marked experimental
for this release. The removed content is commented out with a TODO so it
can be restored once the page leaves experimental.

Related to [internal Slack
thread](https://codercom.slack.com/archives/C04M9CNGHGF/p1790190171687059?thread_ts=1790190121.005749&cid=C04M9CNGHGF).

## Changes

- Comment out the **User spend** page in the dashboard visibility table
and spend details lists in `cost-controls.md` and `spend-management.md`.

> [!NOTE]
> Generated by Coder Agents on behalf of @ssncferreira.

docs/ai-coder/agents/platform-controls/spend-management.md

@@ -55,8 +55,10 @@ Both appear only when the deployment has the AI Gateway entitlement.
 
 Spend is shown where it is actionable:
 
+<!-- TODO(AIGOV-685): Restore once the User spend page moves out of experimental.
 - **Admin settings** > **AI** > **User spend**: total and per-user AI Gateway spend for the selected organization and reporting period.
   If you can also view AI sessions, you can filter it by provider, model, and client.
+-->
 - **Agents page and user menu**: the signed-in user's spend against their budget, as described previously.
 - **Group settings**: each member's spend against the group's budget, for admins who can manage the group.
 - **Chat summary panel**: the cost of one chat tree, on a chat's Summary tab.

docs/ai-coder/ai-gateway/cost-controls.md

@@ -314,11 +314,15 @@ Visibility follows the viewer's role:
 | Every user                                           | Their own spend and budget, or unlimited state, in their avatar menu |
 | Members of a group                                   | The group's spend and budget, and their own member row               |
 | Owners, User Admins, and organization administrators | Spend and budgets for every group and every member                   |
+
+<!-- TODO(AIGOV-685): Restore once the User spend page moves out of experimental.
 | Readers of an organization's group members           | Organization totals and per-user spend on the **User spend** page    |
 
 - The **User spend** page under **Admin settings** > **AI** shows total and per-user AI Gateway spend for the selected organization and reporting period.
   You can open it if you can read the organization's group members, as Owners, Auditors, Template Admins, User Admins, and their organization-scoped equivalents can.
   If you can also view AI sessions, you can filter it by provider, model, and client.
+-->
+
 - The **Groups** page compares each group's spend with the combined limits of
   the members it covers.
 - The **Members** tab of a group reports each member's spend, their budget, and