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/mermaid.md

@@ -1,8 +1,8 @@
----
-title: Mermaid Diagrams
-description: Build-time diagram rendering with Mermaid
-weight: 4
----
++++
+title = "Mermaid Diagrams"
+description = "Build-time diagram rendering with Mermaid"
+weight = 4
++++
 
 sukr renders Mermaid diagrams at build time, producing inline SVG. No client-side JavaScript required.