blog: add Supabase Compute private alpha link to build anything article (#51240)

## Problem

I struggled to find the private alpha form link in the Build Anything
article. I personally expected the link to be at the bottom of the
compute section.

## Solution

Added a link at the bottom of the compute section.

## Checklist

Check all before review:

- [x] I have read
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
- [x] If I wrote a new docs topic or edited an existing topic, I used
the `/write-the-docs` or `/edit-the-docs` skill, which applies the docs
[style
guide](https://github.com/supabase/supabase/tree/master/apps/docs/style-guide)

apps/www/_blog/2026-10-02-select-2026-build-anything.mdx

@@ -48,6 +48,8 @@ Deploy from the Supabase CLI, or manage services through the Management API. The
 
 Each service gets a public HTTP URL, like an Edge Function. Set the number of instances, and Supabase balances requests across them. You can also deploy a service as private, with no HTTP endpoint, for background jobs from a database queue or as a sandbox for an agent. To tighten security further, network restrictions can limit database access to your Compute instances.
 
+Join the waitlist for private alpha [here](https://supabase.com/compute).
+
 ## Your app's MCP server
 
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