docs: add The Stack overview page to Get Started nav

[ig-shaun]

Summary

• Adds the-stack.mdx as a Mintlify-conformant high-level overview of the IXO Stack (IXO Protocol as the verifiable state layer + Qi Intelligent Cooperating System as the human–AI cooperation layer), with sentence-case headings, callouts, an Accordion-driven trust-architecture and core-building-blocks model, and a real Markdown deployment-layers table that points to each canonical home page (/protocols/ixo-protocol, /articles/ixo-graph, /articles/pods, /articles/qi-intelligent-cooperating-system, /articles/ixo-oracles, /articles/ixo-matrix, /articles/ixo-blocksync, /reference/networks-and-endpoints, /articles/ixo-integrations).
• Wires the new page into docs.json under Overview → Get Started, directly below core-concepts, so readers reach a stack-level orientation immediately after the conceptual vocabulary.
• No duplication with introduction.mdx or core-concepts.mdx — those pages remain the canonical sources and are linked from this overview.

Documentation-standards conformance

• Canonical naming per .codex/NAMING_TAXONOMY.md: IXO Protocol, Qi Intelligent Cooperating System, POD (Programmable Organisational Domain), Agentic Oracles, IXO Graph, IXO Matrix, IXO Blocksync, Qi Flows, Model Context Protocol (MCP) servers.
• Sentence-case headings throughout (e.g. The two layers, Core building blocks, Trust architecture, Typical deployment layers).
• Font Awesome icons (matches icons.library: fontawesome in docs.json).
• DIDs expanded once on first mention; external standards links added (W3C VC, W3C DID Core, W3C Data Integrity, UCAN, ZCAP-LD, Matrix Protocol).
• Removed raw text-art separators (⸻) and stray glyphs () and replaced the malformed tab-separated layer list with a real three-column table.

Files changed

• the-stack.mdx (new)
• docs.json — adds the-stack to Overview → Get Started below core-concepts

Test plan

• Mintlify preview renders /the-stack with: Info callout, two-card The two layers group, four Accordion building blocks, four-row trust architecture, deployment-layers table, what-you-can-build cards, design principles, FAQ, next-steps cards.
• Sidebar shows The Stack under Overview → Get Started, between Core concepts and the next group.
• All internal links on /the-stack resolve (already verified against repo routes): /introduction, /core-concepts, /guides/what-you-can-build, /protocols/ixo-protocol, /articles/ixo-graph, /articles/qi-intelligent-cooperating-system, /articles/pods, /guides/users/build-a-pod, /guides/users/build-a-flow, /articles/ixo-oracles, /guides/ixo-oracles-architecture, /articles/claim-evaluation-protocol, /articles/identity-and-credentials, /guides/dev/authentication, /guides/dev/session-keys, /guides/dev/authz, /articles/ixo-matrix, /articles/ixo-blocksync, /articles/ixo-integrations, /guides/digital-mrv, /guides/users/build-obf, /guides/dev/agent-evaluations, /guides/dev/domain-privacy, /mcp/model-context-protocol, /reference/networks-and-endpoints, /reference/product-and-sdk-map, /reference/glossary, /guides/dev/overview.
• No broken links reported by Mintlify build.

Open follow-up (not in this PR)

The top-level Stack tab in docs.json (APIs, Reference, SDKs) shares the name with this new Get Started page. If naming separation is desired, the tab can be renamed (e.g. to Reference) in a follow-up so The Stack in Get Started is the only place that name appears in the sidebar.

Tracked on Linear: World-Class Documentation status update project-update-4b531924.