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.

README.md

@@ -20,6 +20,7 @@ A tiny, Hugo-flavored PHP micro-framework. Routes are directories on disk, pages
 - **SQLite/MySQL database, zero setup** — `Lib\Db`, a thin PDO wrapper (no ORM) supporting multiple named connections open at once — e.g. this site's own SQLite data plus a MySQL connection to a legacy database, usable in the same sidecar — each with its own plain-SQL migration convention, applied automatically on first use or via `php novaconium/bin/migrate.php`. SQLite data lives in a project-owned top-level `data/` directory, outside both `public/` and `novaconium/`. See `/admin/docs/database`.
 - **Sessions with flash data** — `Lib\Session`, a thin wrapper around native PHP sessions with CodeIgniter-style flash values (set now, readable on exactly the next request) for post/redirect/GET flows without a query-string flag. Lazy-start, same mechanism `Lib\Csrf` already uses. See `/admin/docs/session`.
 - **Content index: sitemap, search, tags** — `/sitemap.xml`, full-text `/search` (SQLite FTS5), and blog tag browsing all share one crawler that renders every page and harvests `keywords`/`tags`/`changefreq`/`priority` Twig blocks via Twig's own `renderBlock()` — no front-matter, no separate metadata files. Off by default (depends on SQLite); reindexes lazily on demand or via `php novaconium/bin/index-content.php`. See `/admin/docs/content-index`.
+- **Blog RSS feed** — `/blog/feed`, built from the same hand-written post list `App/pages/blog/index.php` itself renders from, so it works with no database at all. `Lib\Rss` (a small RSS 2.0 envelope builder) also backs a per-tag feed, `/blog/tag/<tag>/feed`, once the content index above is enabled. Auto-discovered via a `<link rel="alternate">` on `/blog/*` pages.
 - **No build step, no Composer** — clone it, point Apache (or `php -S`) at `public/`, and it runs. Twig is vendored as source; see `/admin/docs/upgrading-twig` for upgrading it.
 
 ## Getting started
@@ -80,7 +81,7 @@ php novaconium/bin/create-static-page.php blog/my-new-post
 
 ## Documentation
 
-The full framework documentation — routing, sidecars, libraries, database, session, content index, layouts, static caching, SEO, Matomo analytics, admin authentication, draft pages, styling, project layout, and third-party notices — lives at `/admin/docs` on any running instance (so it travels with the code, no internet connection needed). Highlights:
+The full framework documentation — routing, sidecars, libraries, database, session, content index, XML sitemap, RSS feeds, layouts, static caching, SEO, Matomo analytics, admin authentication, draft pages, styling, project layout, and third-party notices — lives at `/admin/docs` on any running instance (so it travels with the code, no internet connection needed). Highlights:
 
 - [Getting started](http://127.0.0.1:8000/admin/docs/getting-started)
 - [Routing](http://127.0.0.1:8000/admin/docs/routing)
@@ -89,6 +90,8 @@ The full framework documentation — routing, sidecars, libraries, database, ses
 - [Database](http://127.0.0.1:8000/admin/docs/database)
 - [Session](http://127.0.0.1:8000/admin/docs/session)
 - [Content index](http://127.0.0.1:8000/admin/docs/content-index)
+- [XML sitemap](http://127.0.0.1:8000/admin/docs/sitemap)
+- [RSS feeds](http://127.0.0.1:8000/admin/docs/rss)
 - [Layouts](http://127.0.0.1:8000/admin/docs/layouts)
 - [Static caching](http://127.0.0.1:8000/admin/docs/caching)
 - [SEO](http://127.0.0.1:8000/admin/docs/seo)