Add 'Integrate with Mina' top-level section with SDK landing pages

[dkijania]

Part of #1195 (Track 1).

Goal

Introduce a new top-level Integrate with Mina section in the sidebar to give integrators (wallets, indexers, exchanges, bridges, services) a canonical entry point. Today there is none — mina-signer floats at the docs root, the various SDKs (mina-sdk-rust, mina-sdk-go, mina-sdk-python, plus the upcoming mina-archive-sdk-js / mina-archive-sdk-go) have no presence in docs2 at all, and Exchange Operators is its own silo despite being a use case built on Rosetta + archive + signer.

Why a fixed-template SDK page matters

Per-SDK pages should follow an identical skeleton — Install / Quickstart / Auth / Common operations / Link to autogenerated reference / Repo + issues. Predictable URL structure and predictable section order is what makes the section navigable by both humans and AI agents (an agent that has read one SDK page knows how to read all of them).

The actual API reference stays autogenerated in each SDK's own repo (rustdoc, godoc, sphinx). docs2 hosts narrative + canonical link only.

Proposed structure

docs/integrate/
  index.mdx                       # overview + decision tree: which SDK for what
  sdks/
    overview.mdx                  # matrix: language × capability × status
    rust.mdx                      # mina-sdk-rust
    go.mdx                        # mina-sdk-go
    python.mdx                    # mina-sdk-python
    archive-js.mdx                # mina-archive-sdk-js
    archive-go.mdx                # mina-archive-sdk-go
  graphql-api/                    # placeholder — populated by sibling issue
  archive-node-api/               # REST/GraphQL surface of archive-node-api
  rosetta/                        # link or move from exchange-operators
  webhooks-and-streaming/         # subscriptions, future placeholder

Sidebar entry added in sidebars.js between zkApp Developers and Node Developers, or wherever discoverability is best.

Acceptance criteria

• New docs/integrate/ tree with index.mdx and the SDK pages above (stubs are fine for SDKs not yet released — mark status in the matrix).
• Sidebar entry in sidebars.js titled Integrate with Mina.
• All SDK pages share the same section skeleton (Install / Quickstart / Auth / Common operations / Reference link / Repo + issues).
• sdks/overview.mdx contains a capability matrix (language × {accounts, signing, transactions, archive queries, streaming} × {planned, alpha, stable}).
• mina-signer either moved into the new tree or cross-linked from it.
• static/llms-full.txt regenerated and committed (CI requirement).

Open question

Does Exchange Operators stay separate or fold under Integrate as a use-case page composing Rosetta + archive SDK + signer? Recommend folding, but leave the top-level entry redirecting to the new path so existing bookmarks keep working.

Cross-cutting bonus (small, fold in here if cheap)

Add a top-level llms.txt index pointing to per-section landing pages so AI agents can navigate without scraping the full bundle. Sibling to the existing generated llms-full.txt.