docs: add MongoDB migration guide from v6 to Prisma Next (#8111)

* docs(guides): add mongo migration guide from v6 to next

* improve information order

* wrap examples in tabs

* Improve sections 3e and beyond

* remove leftover  md file

* Validate guide against prisma-next 0.16.0; fix adoption flow, ObjectId filters, and raw-command mapping

Every command and snippet now runs against a live v6 database on
mongodb-memory-server (mongod 8.0.4 replica set) migrated to
@prisma-next/mongo@0.16.0. Changes that came out of that run:

- Step 4: db init refuses a populated v6 database (validator adds are
  classed destructive), and migration plans start from an empty baseline
  without a ref, so the adoption path is db update --advance-ref db,
  with plan/migrate/verify for every change after that.
- 3c: the typed pipeline builder passes ObjectId values through as
  strings, so match on an ObjectId field silently returns nothing;
  grouped example no longer filters by authorId and a warning shows the
  working rawCommand + ObjectId pattern.
- 3e/quick reference: $runCommandRaw maps to sharing one MongoClient
  with the mongo() binding (mongoClient + dbName), shown end to end;
  collection-scoped escapes point at db.query.rawCommand.
- Checklist: restore same-database and MongoDB 8.0+ items; rehearsal
  commands now match the adoption flow.
- Step 2: note that every discriminator value in existing data needs a
  declared variant with at least one field.
- Align snippets with the actual init scaffold (contract.d import,
  non-null env assertions), storage-value enum row in common mistakes,
  relative docs links, em-dash cleanup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Ankur Datta <64993082+ankur-arch@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

apps/docs/content/docs/guides/next/meta.json

@@ -3,6 +3,7 @@
   "pages": [
     "index",
     "frameworks",
-    "runtimes"
+    "runtimes",
+    "upgrade-prisma-orm"
   ]
 }

apps/docs/content/docs/guides/next/upgrade-prisma-orm/meta.json

@@ -0,0 +1,6 @@
+{
+  "title": "Upgrade Prisma ORM",
+  "pages": [
+    "mongodb"
+  ]
+}