fix: de-duplicate sidebar and document site structure

[cner-smith]

What does this change?

Two related changes to the website:

1. Fixes duplicated sidebar entries introduced in initial structure upload with placeholders #9, and makes the duplication structurally hard to reintroduce. In site/astro.config.mjs, the Guides / Templates / Tools / Case Studies autogenerate blocks were listed twice, and the Guides autogenerate recursed into the module-0/ and phase-*/ subfolders that were also curated by hand — so every module and phase page was emitted multiple times in the left navigation.

   The fix removes the duplicate block and drives each Module 0 / Phase group from its folder with autogenerate (keeping a custom label) instead of hand-listed slugs. Ordering now comes from each page's existing sidebar.order frontmatter, so the rendered nav is identical (verified: 18 links, no duplicates, original teaching order) — but contributors never edit this config to add, remove, or reorder a page within a section. Guides stays an explicit landing + reference list so it doesn't re-list the module/phase tree.

2. Adds site/README.md — a structure-and-authoring reference covering the two page types, the nested src/content/docs/docs/ folders, the file→URL mapping, how to add a page, and how the sidebar is configured (autogenerate-driven, plus the overlap that produces duplicates). CONTRIBUTING.md links to it.

Why?

The navigation listed the same module and phase pages several times. The root cause was a structural overlap — curated sidebar groups living inside a directory that was also autogenerated. Beyond fixing the symptom, this moves the curated groups onto autogenerate too, so adding content no longer means editing the sidebar config (the step where the overlap was introduced). The new docs explain the layout and the pitfall.

Type of change

• New content (case study, implementation guide, policy template, etc.)
• New code (Terraform, CloudFormation, scripts, etc.)
• New legal template
• Correction or update to existing content
• Documentation or formatting fix
• Other (describe above)

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Checklist

• All factual claims are sourced and citations are included — n/a (config + internal docs)
• Content about a specific Tribal Nation is sourced from that nation's own materials or direct consultation — n/a
• Any pricing or platform-specific details include an "as of [date]" qualifier — n/a
• No vendor promotional content presented as neutral guidance
• If adding code: it has been tested and reviewed for security issues — npm run build passes clean (22 pages, no warnings); built sidebar verified to contain 18 links with no duplicates
• CHANGELOG.md has been updated — handled automatically by release-please from the fix: commit

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Notes for reviewers

• No visible change to the rendered sidebar — the Module 0 / Phase 1–3 layout and ordering are preserved exactly. The change is in how those groups are populated (folder + frontmatter instead of hand-listed slugs).
• Guides is intentionally the one explicit list (landing + example-guide): autogenerating docs/guides would recurse into the module-0/ and phase-*/ folders and re-list them. The rule is documented in site/README.md: each directory is owned by exactly one sidebar entry.
• Two commits: the de-duplication fix, then the refactor onto autogenerate.