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.

templates/page.html

@@ -0,0 +1,10 @@
+{% extends "base.html" %}
+
+{% block content %}
+<article class="page">
+    <h1>{{ page.title }}</h1>
+    <section class="content">
+        {{ content | safe }}
+    </section>
+</article>
+{% endblock content %}