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/_index.md

@@ -1,7 +1,7 @@
----
-title: sukr
-description: Minimal static site compiler — suckless, Rust, zero JS
----
++++
+title = "sukr"
+description = "Minimal static site compiler — suckless, Rust, zero JS"
++++
 
 # sukr