feat: add TOML site config for metadata

- Cargo.toml: Add toml and serde dependencies
- site.toml: New config with title, author, base_url
- src/config.rs: SiteConfig struct with load() function
- src/error.rs: Add Error::Config for parse errors
- src/main.rs: Load site.toml, thread config and page paths
  through generators
- src/templates.rs: Use config.title in nav/titles,
  config.author in footer, config.base_url for canonical URLs.

All three config fields verified in generated HTML output.

site.toml

@@ -0,0 +1,5 @@
+# Site configuration for nrd.sh static site compiler
+
+author   = "nrdxp"
+base_url = "https://nrd.sh/"
+title    = "nrd.sh"