docs: add IS_MULTIWORKSPACE_ENABLED documentation (#16758)

## Summary

Adds comprehensive documentation for the `IS_MULTIWORKSPACE_ENABLED`
configuration variable, which was previously undocumented despite being
a fundamental configuration option that significantly changes how Twenty
behaves.

## Changes

### Self-Host Setup Guide (`setup.mdx`)

Added a new **Multi-Workspace Mode** section that explains:

- **Single-workspace mode (default)**: Only one workspace allowed, first
user gets admin privileges, signups disabled after first workspace
- **Multi-workspace mode**: Multiple workspaces with subdomain-based
URLs (e.g., `sales.your-domain.com`)
- **Related config variables**: `DEFAULT_SUBDOMAIN` and
`IS_WORKSPACE_CREATION_LIMITED_TO_SERVER_ADMINS`
- **DNS configuration**: Wildcard DNS setup for dynamic subdomains
- **Workspace creation restrictions**: How to limit workspace creation
to server admins

### Local Setup Guide (`local-setup.mdx`)

Added an info callout in the environment variables section to make
contributors aware of multi-workspace mode, useful when testing
subdomain-based features.

## Why This Matters

The `IS_MULTIWORKSPACE_ENABLED` variable controls:
- Whether multiple workspaces can exist on a single instance
- URL structure (plain domain vs subdomains)
- First user privileges
- Sign-up behavior after initial setup
- SSO workspace resolution logic

This is critical knowledge for self-hosters who want to run Twenty as a
multi-tenant SaaS.

packages/twenty-docs/developers/contribute/capabilities/local-setup.mdx

@@ -235,6 +235,10 @@ cp ./packages/twenty-front/.env.example ./packages/twenty-front/.env
 cp ./packages/twenty-server/.env.example ./packages/twenty-server/.env
 ```
 
+<Info>
+**Multi-Workspace Mode:** By default, Twenty runs in single-workspace mode where only one workspace can be created. To enable multi-workspace support (useful for testing subdomain-based features), set `IS_MULTIWORKSPACE_ENABLED=true` in your server `.env` file. See [Multi-Workspace Mode](/developers/self-host/capabilities/setup#multi-workspace-mode) for details.
+</Info>
+
 ## Step 6: Installing dependencies
 To build Twenty server and seed some data into your database, run the following command:
 ```bash

packages/twenty-docs/developers/self-host/capabilities/setup.mdx

@@ -59,6 +59,61 @@ IS_CONFIG_VARIABLES_IN_DB_ENABLED=false
 3. Restart containers for changes to take effect
 4. Admin panel will show current values but cannot modify them
 
+## Multi-Workspace Mode
+
+By default, Twenty runs in **single-workspace mode** — ideal for most self-hosted deployments where you need one CRM instance for your organization.
+
+### Single-Workspace Mode (Default)
+
+```bash
+IS_MULTIWORKSPACE_ENABLED=false  # default
+```
+
+- One workspace per Twenty instance
+- First user automatically becomes admin with full privileges (`canImpersonate` and `canAccessFullAdminPanel`)
+- New signups are disabled after the first workspace is created
+- Simple URL structure: `https://your-domain.com`
+
+### Enabling Multi-Workspace Mode
+
+```bash
+IS_MULTIWORKSPACE_ENABLED=true
+DEFAULT_SUBDOMAIN=app  # default value
+```
+
+Enable multi-workspace mode for SaaS-like deployments where multiple independent teams need their own workspaces on the same Twenty instance.
+
+**Key differences from single-workspace mode:**
+- Multiple workspaces can be created on the same instance
+- Each workspace gets its own subdomain (e.g., `sales.your-domain.com`, `marketing.your-domain.com`)
+- Users sign up and log in at `{DEFAULT_SUBDOMAIN}.your-domain.com` (e.g., `app.your-domain.com`)
+- No automatic admin privileges — first user in each workspace is a regular user
+- Workspace-specific settings like subdomain and custom domain become available in workspace settings
+
+<Warning>
+**Environment-only setting:** `IS_MULTIWORKSPACE_ENABLED` can only be configured via `.env` file and requires a restart. It cannot be changed through the admin panel.
+</Warning>
+
+### DNS Configuration for Multi-Workspace
+
+When using multi-workspace mode, configure your DNS with a wildcard record to allow dynamic subdomain creation:
+
+```
+*.your-domain.com -> your-server-ip
+```
+
+This enables automatic subdomain routing for new workspaces without manual DNS configuration.
+
+### Restricting Workspace Creation
+
+In multi-workspace mode, you may want to limit who can create new workspaces:
+
+```bash
+IS_WORKSPACE_CREATION_LIMITED_TO_SERVER_ADMINS=true
+```
+
+When enabled, only users with `canAccessFullAdminPanel` can create additional workspaces. Users can still create their first workspace during initial signup.
+
 ## Gmail & Google Calendar Integration
 
 ### Create Google Cloud Project