Ingest Miden Up docs from miden-up repo into docs.miden.xyz

Source: DevEx Sync (Notion).

The Miden Up content authored for the Lambda class PR has not yet landed in `docs.miden.xyz`. Currently Miden Up docs are sparse and not integrated into the new v0.14 IA (Get Started / Tools / Tutorials).

Proposed approach (per discussion):

- Create a `docs/` folder inside the `miden-up` repo and have `docs.miden.xyz` ingest it (mirroring how the Rust client docs are pulled from `miden-client/`), instead of authoring a single giant page in this repo.
- Land the existing Lambda-class Miden Up walkthrough as the first ingested content.
- Extend the docs deploy workflow (`.github/workflows/deploy-docs.yml`) to whitelist the new path.

Owner per meeting: Philipp.

Upstream dependency: requires a `docs/` folder + structure to be set up in the `miden-up` repo first; this issue tracks the docs-side ingestion + IA placement.