docs(branching): note admin requirement for first branch in dashboad (#46471)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

  Docs update

  ## What is the current behavior?

The dashboard branching guide doesn't mention that the first "Create
branch" action on a project requires Owner/Administrator access.
Combined with cosmetic UI behavior: the branch dropdown displays `main /
PRODUCTION` on every project from day one, even though the backend
hasn't registered a production branch yet. Developer-role users hit a
403 with a generic "Not authorized to enable preview branching" toast on
their first branch attempt, with no docs or UI hint that an implicit
production-branch registration step is happening.

Customer-reported in
[SU-383693](https://supabase.frontapp.com/open/cnv_1mz95u3i?key=wbyHQkruuZ_KYRGnRPB5_yHzm5-fr_k4)

  ## What is the new behavior?

Adds an admonition under the "Creating a branch" heading in the
dashboard-branching guide explaining:
  
- The cosmetic UI vs. backend state mismatch (`main / PRODUCTION` shown
before any row exists)
- That the first "Create branch" click is what technically enables
branching on the project (populates `preview_branches` with`is_default =
true`)
  - This step requires Owner/Administrator access
- After that one-time step, anyone with the Developer role can create,
update, and delete preview branches

Scoped to the gitless dashboard flow only. The GitHub-integrated path
bootstraps automatically during admin-only integration setup.

  ## Additional context
  
- Mechanism confirmed with the team in
[Slack](https://supabase.slack.com/archives/C02BJ2239GA/p1779978863774239)
"the first time branching is used we populate the preview_branches table
with the base project and have is_default set as true. This is
technically what enables branching. A developer role can't do that."
- Related FE improvement being escalated separately (clearer error
message and/or disabling the Create branch button for Developers when
the production branch hasn't been registered yet)
- The [Branching 2.0
announcement](https://supabase.com/blog/branching-without-git-is-now-the-default)
(May 2026) made gitless branching the default for new projects, and more
customers can walk into the dashboard flow without ever touching GitHub
integration.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added clarification on dashboard branching behavior when GitHub
integration is not configured, including details on branch dropdown
display and initial setup requirements with permission levels.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/docs/content/guides/deployment/branching/dashboard.mdx

@@ -30,6 +30,12 @@ This functionality is currently in beta and requires opting in. To opt in you mu
 
 ## Creating a branch
 
+<Admonition type="note">
+
+When using dashboard branching without a GitHub integration, the branch dropdown displays `main / PRODUCTION` on every project, but on a brand-new project the production branch hasn't been registered yet. The first "Create branch" click populates this row behind the scenes — technically enabling branching on the project — and requires Owner/Administrator access. After that one-time step, anyone with the Developer role can create, update, and delete preview branches.
+
+</Admonition>
+
 Once you've enabled the feature, you can create a new branch:
 
 1. Click the arrows next to the branch name in the top menu bar. (The top menu bar has the format `YOUR_ORGANIZATION / YOUR_PROJECT / CURRENT_BRANCH_NAME`.)