Split Sage and Wallet SDK docs into separate sidebars - #8

Sage and the Wallet SDK serve different readers (people running or integrating with Sage vs. Rust developers building their own apps), and the generated RPC reference was pushing the SDK pages far down a single shared sidebar.

Changes

• sidebars.ts: replace the single sidebar with sage (Getting Started, Files, Config, RPC) and sdk (the Wallet SDK tree, now top-level instead of nested in a category).
• Navbar: replace the "Docs" item with "Sage" and "Wallet SDK" items.
• Homepage: the hero now has "Sage Docs" and "Wallet SDK Docs" buttons instead of a single "Get Started".
• Cross-links: the Getting Started intro is now Sage-specific and points to the SDK docs; the SDK intro links back to Sage.
• Sage docs move to docs/sage/ and are served under /sage/, matching /sdk/. Generated RPC pages now go to docs/sage/rpc/.

URL changes

@docusaurus/plugin-client-redirects redirects every old URL to its new /sage/ location, and /sage redirects to Getting Started. Wallet SDK URLs are unchanged, and search still covers both sections.

Other fixes

• The RPC category now includes the generated introduction page (API version and authentication), which was previously dropped by slice(1) and showed no sidebar.
• docs/rpc/ stays in .gitignore and .prettierignore and is excluded from the docs plugin, so RPC pages generated locally before this change aren't built or formatted alongside the new ones.

Testing

• pnpm format:check, pnpm typecheck, pnpm build pass (build uses onBrokenLinks: "throw").
• Checked the built redirect pages for /sage, /getting-started, /config, /files, /rpc/setup and /rpc/send-xch, and that a stale docs/rpc page is not built.