[Website] Add Books/Talks, Reference Reading and Professional Support pages; split Powered By into Commercial and Academic Use

[400Ping]

Summary

Follow-up to #1493. Restores three pages from the pre-2023 site and reworks the Powered By page layout.

Restored pages (under Resources)

Recovered from git history (last Jekyll-era versions) rather than retyped, so the original links are intact. Placed in docs/learning/ next to the Primer and Research Papers, and exposed via the Resources navbar dropdown and sidebar category.

• Books, Tutorials and Talks (learning/books-tutorials-and-talks): Books, News/Articles/Tutorials, Coursework/Lectures, and Talks, ~100 entries. Each section now leads with a Qumat and QDP subsection (Primer, Research Papers, the 0.4/0.5/0.6 release posts, the two quickstarts, and the Community Over Code Asia 2026 / FOSDEM 2025 / FOSSY 2024 talks) followed by the Mahout Classic material.
• Reference Reading (learning/reference-reading): the background reading list the old wiki linked under "Background Material".
• Professional Support (learning/professional-support): people/companies for hire and speakers, from the last published version, plus a note that Classic-era contact details may be stale. Adds Jie-Kai Chang to both tables.

All three are mirrored into the 0.6 snapshot (and its sidebar) because navbar doc items resolve against the latest release.

Powered By

• Page now has two sections, Commercial Use (33) and Academic Use (7), alphabetical within each, matching the original page. Entries carry a type field in powered-by.json.
• Every entry has a one-line description taken from the original page (previously only 18 did). Six organizations that were on the Confluence version but not the Jekyll one were added: Amazon, Cull.tv, Evolv, Mippin, Next Glass, NoTube project.
• Uniform card layout: a fixed 96px white logo box at the top of every card (56px-high logo, name as text fallback), so cards line up regardless of logo shape.
• The old "featured cards + logo wall" layout and the Powered By badge section are removed.