docs(auth): add server-side package disambiguation guide (#47966)

Adds a docs guide, "Choosing a server-side package", that explains when
to use `supabase-js`, `@supabase/ssr`, or `@supabase/server` when
working with Supabase from JavaScript on the server. It includes a
decision table and a short code example for each, with one rule up
front: cookie-based sessions in SSR frameworks use `@supabase/ssr`,
per-request header auth in Edge Functions and other backend runtimes
uses `@supabase/server`, and `supabase-js` is the base client both wrap.

The guide is surfaced from the Auth overview page and the sidebar, and
is cross-linked from the `supabase-js` and `@supabase/server` reference
introductions so it is reachable from where developers start. It also
states that the packages coexist and are not replacements for each
other, and keeps combining `@supabase/server` with `@supabase/ssr` as an
advanced section.

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

- **New Features**
- Added a guide explaining how to choose between Supabase server-side
JavaScript packages.
- Added the guide to Auth navigation and the getting-started content
listings.
- Added links to the new guidance throughout relevant JavaScript and
server documentation.
- **Documentation**
- Clarified when to use cookie-based sessions versus header-based
authentication.
- Added package comparisons, usage examples, advanced guidance, and
related next steps.
- Updated spelling support for framework names used in the
documentation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

apps/docs/spec/reference/javascript/v2/partials/introduction.mdx

@@ -6,3 +6,5 @@ title: Introduction
 This reference documents every object and method available in Supabase's isomorphic JavaScript library, `supabase-js`. You can use `supabase-js` to interact with your Postgres database, listen to database changes, invoke Deno Edge Functions, build login and user management functionality, and manage large files.
 
 To convert SQL queries to `supabase-js` calls, use the [SQL to REST API translator](/docs/guides/api/sql-to-rest).
+
+Using `supabase-js` on the server? See [which package to use](/docs/guides/auth/choosing-a-server-package) to decide between `supabase-js`, `@supabase/ssr`, and `@supabase/server`.

apps/docs/spec/reference/server/v1/partials/introduction.mdx

@@ -6,3 +6,9 @@ title: Introduction
 `@supabase/server` is a framework-agnostic library for authenticating requests in server-side JavaScript environments. It verifies JWTs, resolves Supabase API keys, and creates pre-configured Supabase clients — exposing everything through a single `SupabaseContext` that is identical regardless of which adapter or primitive produced it.
 
 Adapters for Hono, H3, Elysia, and NestJS are included. You can also compose the lower-level primitives directly for custom frameworks or edge runtimes.
+
+<Admonition type="note" title="Not sure this is the right package?">
+
+See [which package to use](/docs/guides/auth/choosing-a-server-package) — for cookie-based sessions in SSR frameworks, use [`@supabase/ssr`](/docs/guides/auth/server-side) instead.
+
+</Admonition>