Publish an Activity RSS feed at /activity/feed.xml

[mmcky]

Follows the Activity RSS feed decision in #266. Related to #271, but not one of its work items.

Why. Decided in #266: a second feed for Activity. It gives one subscription covering every library release and lecture update, which GitHub's per-repository release feeds don't. /feed.xml stays News-only.

Work.

• The feed page. Add pages/activity-feed.xml (layout: null, permalink: /activity/feed.xml, sitemap: false), built from Compute the Activity rows at build time, with tests #276's rows:
  • the latest 50 items, in RSS 2.0 to match /feed.xml;
  • each item's guid built from date, type and project, since lecture URLs repeat;
  • each item links to its day on /activity/.
• Discovery. Add <link rel="alternate" type="application/rss+xml"> for both feeds in _layouts/default.html.
• Links. If the rail has shipped, turn on its "Activity feed" link. Otherwise the rail adds it when it ships.

Acceptance criteria

• /activity/feed.xml validates with the W3C feed validator.
• Its newest item matches the top of /activity/.
• /feed.xml is unchanged.
• Both feeds are advertised in <head>.

[mmcky]

Feed rules (decided 2026-09-29). These are hard to change once people subscribe.

• One item per row of Compute the Activity rows at build time, with tests #276's view: the newest 50 rows.
• Item ID: the row ID from Compute the Activity rows at build time, with tests #276 (its grouping key: date, type, and series or release group), as a tag: URI with isPermaLink="false". It must not change once published.
• Titles start with the row's type word, e.g. "Release: QuantEcon.py v0.12.0"; book rows read "Book update: …".
• Dates: 12:00 UTC on the entry's date. Don't use date_to_rfc822: the site runs on Sydney time, so it would shift every entry back a day.
• Launch after the replay of the paused days, so replayed entries don't reach subscribers as a burst of new items.
• The PR description says "Related to News and Activity: announcements on News and a self-updating Activity feed #271", since this issue is deliberately unparented.

[mmcky]

Unblocked (2026-10-01). The replayed days are published: #293 added the feed's first lecture entry (2026-09-29), so launching the feed won't announce the paused days as a burst of new items. The rail is live (#291), so the PR for this issue adds the rail's "Activity feed" link (bi-rss) after "View all activity", as #291's notes say.

One thing to settle when planning: the June to September backfill (QuantEcon/reports-activity#91) will later add older rows. Feed readers would show them as new, unread items. Either launch after the backfill, or accept that.