Merge the News and Activity split #273

Part of #271 (Phase 1).

What it lands. #265 moves the reporter's output off News and gives Activity its own data and page:

• The data. 40 Activity entries in 23 files under _data/activity/ (24 releases, 12 lecture updates, 4 translation updates): 35 migrated from the bot's 11 merged posts and 14 drafts, plus 5 library releases the bot never reported. Each entry was checked against GitHub.
• The page. An interim /activity/ page.
• The rules. The stream definitions and the data schema, in the README's "News and Activity" section.
• CI. A data check, .github/scripts/check-activity-data.rb, run in build.yml.
• Redirects from the 11 removed post URLs to /activity/.
• Navigation. Activity is treated as part of News, with no menu item of its own.

Where it stands. Verified 2026-09-29 at head 44f5479. It merges cleanly with main (44a38e9), CI and the Netlify preview pass, and it is blocked only on the one required approval.

Acceptance criteria

• Split updates into News and an Activity feed #265 is merged.
• https://quantecon.org/activity/ lists the 40 entries under September to June 2026.
• https://quantecon.org/2026/07/15/software.html redirects to /activity/ (spot-check two more of the 11).
• /news/, the home page's "Latest News" and /feed.xml carry no bot posts.
• build.yml on main includes the data check, and the next PR to main shows its "Check Activity data" step passing. The push-to-main deploy (deploy.yml) doesn't run the check.