Improve MASM docs discoverability and depth in IA

Source: DevEx Sync (Notion).

MASM reference documentation currently lives under Core Concepts / Protocol but is not intuitive to find for developers writing MASM. Two options were discussed:

1. Stand up a dedicated Miden-flavoured MASM section (e.g. top-level `/masm/` or under Tools), authored in this repo.
2. Update the docs ingestion logic so MASM reference pages from the upstream source are pulled into a more discoverable location with proper sidebar grouping.

Additionally:

- Several MASM pages (and project-template pages) are currently single-page stubs and need deeper content per page rather than one wall.
- Sidebar/IA placement should make the MASM reference reachable in ≤ 2 clicks from the home page.

This issue tracks both the IA decision and the resulting content/ingestion work.