docs: serve a docs home page at /docs

[a-essawy]

/docs answered with two chained 308s to /docs/quickstart. Every link to docs from the marketing site landed on that chain, and Google had no 200 page to attach a "Docs" sitelink to.

Mintlify serves a folder's index.mdx at the folder itself (storage/index.mdx is /docs/storage), so a root index.mdx serves /docs directly.

Changes

• index.mdx — hub page linking every section, answer-first intro for AI search
• docs.json — index first in Getting Started, or the nav validator flags it as an orphan
• validate-frontmatter.mjs — root index canonical is /docs, not /docs/index
• add-canonical.mjs — strip /index the same way. It never did, so a re-run would have written /docs/storage/index over the two existing folder indexes.

Test plan

All five validators pass locally:

validate:frontmatter   Checked 35 MDX files. All frontmatter valid.
validate:nav           Checked 35 pages, 35 nav entries, 24 redirects. All consistent.
validate:plan-limits   Plan-limit tables and descriptions match the source of truth.
validate:webhooks      36/36 checks passed.
validate:templates     Checked 2 template cards. All consistent.

After merge, verify https://rendobar.com/docs returns 200 rather than 308.