chore: full repo cleanup — remove Jekyll remnants, duplicates, and dead code

- Remove old Jekyll README.adoc (content migrated to README.md)
- Remove completed TODO.redesign/ planning docs
- Remove duplicate BNF files (bnf/, content/bnf/) — kept only public/bnf/
- Remove Jekyll-era .vscode log files and settings
- Remove unreachable src/pages/references.vue (route redirects to /standards)
- Remove unused content/pages/{references,language}.adoc sources
- Remove 9 unreferenced logo variants from public/logos/
- Remove unreferenced favicon-16x16.png and favicon.png
- Remove vite-plugin-sitemap (incompatible with vite-ssg, custom script works)
- Add *.log to .gitignore
- Remove /references from sitemap route list in vite.config.ts

Author: ronaldtse

.gitignore

@@ -11,6 +11,7 @@ src/content
 
 # IDE
 .vscode
+*.log
 
 # Misc
 tmp

.vscode/configurationCache.log

@@ -22,6 +22,20 @@ Content is authored in AsciiDoc under `content/`:
 
 The build pipeline converts `.adoc` files to JSON via `scripts/build-content.ts`, then ViteSSG generates static HTML.
 
+## Content authoring
+
+Content files use AsciiDoc syntax (`.adoc` extension) with YAML frontmatter:
+
+```
+---
+title: Page Title
+---
+
+AsciiDoc content here...
+```
+
+See the [AsciiDoc syntax quick reference](https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/) for markup details. After the frontmatter block, normal AsciiDoc content begins.
+
 ## Deployment
 
 Automatically deployed to GitHub Pages on push to `main` via GitHub Actions.