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/section/default.html

@@ -0,0 +1,15 @@
+{% extends "base.html" %}
+
+{% block content %}
+<h1>{{ section.title }}</h1>
+<ul class="item-list">
+    {% for item in items %}
+    <li>
+        <a href="./{{ item.slug }}.html">{{ item.frontmatter.title }}</a>
+        {% if item.frontmatter.description %}
+        <p>{{ item.frontmatter.description }}</p>
+        {% endif %}
+    </li>
+    {% endfor %}
+</ul>
+{% endblock content %}