Docs: say what the forge cannot render, and two headings by the style guide

The documentation sources are Zola Markdown: front matter between +++
lines, shortcodes that insert the example configurations, and @/ internal
links. A code forge shows all three unrendered, which the README used to
deny. A docs/README.md now explains each, and the README points to it.

"Configuring BGP Neighbors" becomes "BGP Neighbors", as the other feature
pages are titled; "When a Neighbor Restarts" becomes "Neighbor Restart", a
noun phrase as the style guide asks.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -89,9 +89,11 @@ privileges, the systemd unit, and upgrading without withdrawing routes.
 ## Documentation
 
 The documentation is a website built from [`docs/`](docs/) with
-[Zola](https://www.getzola.org) (`zola --root docs serve`, then
-<http://127.0.0.1:1111>). The pages are plain Markdown and can be read
-in the repository:
+[Zola](https://www.getzola.org): `zola --root docs serve`, then open
+<http://127.0.0.1:1111>. The sources are Markdown with Zola front matter,
+shortcodes and internal links, so the forge shows them with those parts
+unrendered; [`docs/README.md`](docs/README.md) explains what each is. The
+pages:
 
 - [Overview](docs/content/docs/getting-started/introduction.md),
   [installation](docs/content/docs/getting-started/installation.md) and