feat(templates): add Tera runtime template engine

Lay groundwork for user-editable templates by adding Tera as a
runtime template engine alongside the existing maud templates.

Changes:
- Add tera dependency
- Create TemplateEngine struct with render methods
- Add TemplateLoad/TemplateRender error variants
- Add section_type/template fields to Frontmatter
- Create templates/ directory with base, page, section, and content templates

Dead code warnings are expected; TemplateEngine will be wired
in to replace maud in subsequent commits.

src/main.rs

@@ -11,6 +11,7 @@ mod highlight;
 mod math;
 mod mermaid;
 mod render;
+mod template_engine;
 mod templates;
 
 use crate::content::{discover_nav, Content, ContentKind, NavItem};