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.

App/pages/blog/feed/index.php

@@ -0,0 +1,48 @@
+<?php
+
+// /blog/feed — sidecar-only (no index.twig), like sitemap.xml/search: no
+// point rendering Twig just to return XML. Project-owned (App/pages/),
+// since this is blog content specifically, not generic framework
+// machinery like sitemap.xml/search are. Deliberately has zero dependency
+// on the (off-by-default) content index — it reads the exact same
+// hand-written $posts array App/pages/blog/index.php itself renders from,
+// so this feed works on a bare install with content_index_enabled left at
+// its shipped default of false. Only the per-tag variant
+// (App/pages/blog/tag/[tag]/feed/index.php) needs the content index, since
+// tags only exist there.
+
+use App\Response;
+use Lib\Rss;
+
+$config = require __DIR__ . '/../../../../novaconium/config.php';
+$appConfigFile = __DIR__ . '/../../../../App/config.php';
+if (is_file($appConfigFile)) {
+    $config = array_merge($config, require $appConfigFile);
+}
+
+$posts = (require __DIR__ . '/../index.php')['posts'];
+
+// Newest first, the RSS convention — the array itself (and therefore the
+// /blog listing page, which isn't touched here) keeps its own order;
+// sorting only affects this feed's output.
+usort($posts, fn (array $a, array $b) => strcmp($b['published'], $a['published']));
+
+$items = array_map(
+    fn (array $post) => [
+        'title'            => $post['title'],
+        'link'             => '/blog/' . $post['slug'],
+        'guid'             => '/blog/' . $post['slug'],
+        'pubDateTimestamp' => strtotime($post['published']),
+        'description'      => $post['excerpt'],
+    ],
+    $posts
+);
+
+$xml = Rss::render(
+    $config['site_name'] . ' Blog',
+    '/blog',
+    'Posts from ' . $config['site_name'] . '.',
+    $items
+);
+
+return Response::xml($xml);