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/_index.md

@@ -1,8 +1,8 @@
----
-title: Features
-description: Explore sukr's capabilities
-section_type: features
-weight: 3
----
++++
+title = "Features"
+description = "Explore sukr's capabilities"
+section_type = "features"
+weight = 3
++++
 
 sukr provides a focused set of features for building fast, minimal static sites.