Restructure public docs around the operator journey

[swarna1101]

Replace the current layout (Overview, Gateway link, Research) with the journey structure from the Notion ticket.

Tasks

• New sidebar in .vitepress/config.mts with these sections, in order: Start here, Part 1 Getting in, Part 2 Signal, Part 3 Accelerate, Part 4 Console & gateway management, Reference, Help, Learn.
• A real landing page in index.md, replacing the JS redirect to /docs/learn/overview/intro. Its primary call to action is "Start in Console".
• Move docs/learn/overview/p2p.md (mump2p) and docs/research/** under Learn. Keep their content unchanged.
• A Reference section that links to the gateway docs through versions/latest/: configuration, Consumer Block Stream, metrics, release notes, security audit, and Obol DVT.
• A redirect from /docs/learn/overview/intro to the new Start page.
• Set Console's NEXT_PUBLIC_DOCS_URL to the Start page. This change is in optimum-cross-functional-dashboard.

Done when

The sidebar matches the journey. Pages that don't exist yet can be placeholders that aren't linked from the landing page.

[linear-code]

OPT-1114