fix(docs): remove duplicate page titles after mystmd conversion

[henryiii]

🤖 AI text below 🤖

Problem

Follow-up to #801. On the mystmd site every page showed its title twice, and the landing page rendered a bare Home heading above the content (review note).

Root cause

The mystmd migration (#792) gave every page a frontmatter title: and demoted the original # page heading to ##. The book-theme renders the frontmatter title as a title-block H1, so the ## heading below it became a visible duplicate. On index.md the title was Home, so the page led with a stray "Home" H1.

Fix

For each page, drop the redundant frontmatter title and promote body headings back up one level. MyST then extracts the single # H1 as the page title (rendered once in the title block), and sections nest correctly beneath it.

• short_title still drives the nav labels on the GHA/style pages; their title block now shows the fuller heading instead of the terse duplicate.
• index.md keeps a short_title: Home nav label while its title becomes Scientific Python Library Development Guide.
• packaging_simple.md / packaging_compiled.md had a tip/cog block before the heading (which blocks MyST title extraction), so their # heading moves to the top.

Verification

• Built with mystmd and walked the rendered mdast for all pages: every page has a correct title and zero body headings duplicating it.
• prek -a passes fully, including rumdl (MD041), blacken-docs, and Cog the pages (the cog reorder in packaging_compiled regenerates cleanly).
• The diff only touches headings and frontmatter — no prose or code changes.

The other review notes are theme work (tab selection, configurable highlight scheme, three-way theme toggle) tracked separately; the site already renders the nav/outline table of contents.

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📚 Documentation preview 📚: https://scientific-python-cookie--805.org.readthedocs.build/