docs: migrate content files to TOML frontmatter

Migrate all 17 docs/content/ files from --- YAML to +++ TOML
frontmatter delimiters and key = value syntax.

Update 8 embedded frontmatter examples in 7 documentation pages
to match (configuration, content-organization, getting-started,
security, sections, sitemap, feeds, templates).

Update configuration.md frontmatter reference table: add draft
and aliases fields, correct date type from string to date.

docs/content/features/math.md

@@ -1,8 +1,8 @@
----
-title: Math Rendering
-description: Build-time LaTeX math with KaTeX
-weight: 5
----
++++
+title = "Math Rendering"
+description = "Build-time LaTeX math with KaTeX"
+weight = 5
++++
 
 sukr renders LaTeX math expressions at build time using KaTeX, producing static HTML and CSS. No client-side JavaScript required.