Add Reference sidebar landing pages to fix breadcrumb navigation

[cderv]

When clicking the Formats breadcrumb on /docs/reference/formats/<page>.html, the link led to formats/html.html (the first child) instead of a section landing. The Reference > More section had a similar issue and was not a coherent grouping for its contents.

Adds explicit landing pages for three Reference sidebar sections so the breadcrumb renders as a clickable link to a real page:

• Formats → new docs/reference/formats/index.qmd
• Projects → new docs/reference/projects/index.qmd
• More → renamed to Metadata and pointed at new docs/reference/metadata/index.qmd

The More → Metadata rename matches the grouping already used in docs/reference/reference.yml (the master listing the website renders) — Dates, Globs, Citations, Cross-References, and Brand are now grouped under Metadata in the sidebar too. The five items themselves are unchanged.

Each landing mirrors the existing docs/reference/cells/ pattern: small index.qmd shell + a per-subdir reference.yml subset rendered by the existing ejs/links.ejs template. No new extension, shortcode, or filter. The master docs/reference/reference.yml is untouched.

Reported in quarto-dev/quarto-cli#14402. The underlying CLI fallback behavior (sidebar sections without href: use contents[0].href) is tracked separately at quarto-dev/quarto-cli#14454 and is not addressed here. Inner Guide sub-sections (HTML/PDF/MS Word/Typst/Markdown under Documents) and Reference > Quarto CLI continue to inherit the same fallback and are deferred.

A follow-up PR will fix the Documents section in the Guide sidebar.

[github-actions]

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🔄 Modified Documents

• docs/reference/formats/index.html
• docs/reference/metadata/index.html
• docs/reference/projects/index.html