Add Blog RSS feed, footer feed/sitemap links, and expanded docs

Blog RSS feed (novaconium/ISSUES.md):
- App/pages/blog/feed/index.php - main feed, built from the same
  hand-written $posts array App/pages/blog/index.php renders from, so it
  works with content_index_enabled left at its default false. Added a
  published date field per post entry.
- App/pages/blog/tag/[tag]/feed/index.php - per-tag feed, gated on
  content_index_enabled the same way blog/tag/[tag]/index.php is.
- novaconium/lib/Rss.php - shared RSS 2.0 envelope builder both feeds
  use, generic (title/link/description/items in, XML string out).
- New head_extra block in the root layout (empty by default) so
  App/pages/blog/_layout/layout.twig can add feed auto-discovery scoped
  to /blog/* only.

Footer:
- New content_index_enabled Twig global (Renderer/bootstrap.php), same
  pattern as admin_auth_enabled.
- Right-aligned footer menu linking to /sitemap.xml (only shown when
  content_index_enabled, so it never links to a 404) and /blog/feed
  (always shown, no content-index dependency).

Docs:
- New /admin/docs/rss page: Lib\Rss API, the two shipped feeds, a worked
  example of adding a feed for another content collection, and how to
  advertise multiple feeds via head_extra.
- New /admin/docs/sitemap page: changefreq/priority blocks, what's
  included/excluded, an honest callout on the relative-<loc>-URL spec
  deviation (consistent with how canonical/og:url already work) with an
  override path for strict compliance, and submitting it to search
  engines.
- /admin/docs (Overview) gains a Requirements section: minimum
  requirements, then optional requirements for the database/content-index
  features (pdo_sqlite, optional pdo_mysql, FTS5) - and a Documentation
  heading separating it from the doc links list.
- /admin/docs/getting-started's Requirements line was stale (missing
  pdo_sqlite entirely, unlike README) - fixed and cross-linked to the
  Overview page's fuller list.
- /admin/docs/content-index trimmed to point at the new RSS/sitemap pages
  instead of duplicating their detail.

Also adds an "In-house comments" entry to novaconium/ISSUES.md's Backlog
- a Lib\ class for sidecar-attached comments on any page, depending on
  the not-yet-built Admin login & user management for real user accounts.

Closes the "Blog RSS feed" backlog item.

public/css/main.css

@@ -194,6 +194,19 @@ small {
   color: var(--muted-color);
 }
 
+footer {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  flex-wrap: wrap;
+}
+
+.footer-menu {
+  display: flex;
+  gap: 1rem;
+}
+
 .hp-field {
   position: absolute;
   left: -9999px;