feat: promote Module 0 and Phases into a top-level Introduction section

[cner-smith]

What

Moves the foundational Module 0 content and the three implementation Phases out of the Guides section into a new, fifth top-level Introduction section — so the orientation material is no longer buried inside Guides.

Top-level nav goes from four sections to five:

Introduction · Guides · Templates · Tools · Case Studies

Introduction sits first as the "start here" orientation section.

Changes

• Moved files: docs/guides/{module-0,phase-1,phase-2,phase-3}/ → docs/introduction/..., giving clean URLs like /docs/introduction/module-0/.
• New landing page for the section: docs/introduction/index.md.
• Sidebar (astro.config.mjs): new top-level Introduction group holding the landing + curated Module 0 / Phase sub-groups. Guides collapses to a plain autogenerate now that its subfolders moved out.
• Fixed broken internal links: the moved pages linked to /docs/module-0/ and /docs/phase-*/ — missing the parent path segment, so they previously 404'd. Now corrected to /docs/introduction/....
• /docs landing page: added an Introduction card and repointed the primary hero action to "Start here" → Introduction.
• site/README.md: updated throughout to document the new structure (folder map, file→URL table, sidebar explanation, the autogenerate duplication gotcha).

Notes

Pure information-architecture change — all page content, intra-folder slugs, and ordering are preserved.

URLs under /docs/guides/module-0/ and /docs/guides/phase-*/ change to /docs/introduction/.... Those pages weren't reachable via working links before (the in-content links were already broken), so external bookmarks are unlikely, but flagging it.

Verification

• npm run build clean — 23 pages, no errors/warnings.
• Parsed the built nav HTML: five top-level groups render in order; Module 0 (Overview + 6 pages) and Phases 1–3 nest under Introduction; Guides shows its landing + example-guide.
• Swept all built HTML: zero stale /docs/guides/module-0, /docs/guides/phase-*, /docs/module-0, or /docs/phase-* references.