RSS: per-tag feeds, an Atom variant, and a JSON Feed mirror

[RichardLitt]

Follow-up from #144, which added a single combined RSS feed of all new patterns. A pre-merge review surfaced three places where the feed coverage could be broader. Splitting this out as its own issue.

• Per-tag feeds — somewhere like /tags/funding/feed.xml so someone who only cares about (say) "Funding & Financial Support" can subscribe to just those patterns instead of the whole catalogue. This is the highest-value of the three, because it lets people self-select.
• Atom feed variant alongside the existing RSS 2.0 feed. The current mkdocs-rss-plugin config doesn't ship Atom; some scholarly readers and indexing tools prefer Atom over RSS.
• JSON Feed mirror at something like /feed.json for modern reader apps (NetNewsWire, Feedbin). These often handle JSON Feed better than RSS or Atom.

None of this changes anything for contributors or breaks the existing /feed_rss_created.xml from #144 — it's purely additive.