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/content/post.html

@@ -0,0 +1,25 @@
+{% extends "base.html" %}
+
+{% block content %}
+<article class="post">
+    <header>
+        <h1>{{ page.title }}</h1>
+        {% if page.date %}
+        <time class="date">{{ page.date }}</time>
+        {% endif %}
+        {% if page.description %}
+        <p class="description">{{ page.description }}</p>
+        {% endif %}
+        {% if page.tags %}
+        <ul class="tags">
+            {% for tag in page.tags %}
+            <li><a href="{{ prefix }}/tags/{{ tag }}.html">{{ tag }}</a></li>
+            {% endfor %}
+        </ul>
+        {% endif %}
+    </header>
+    <section class="content">
+        {{ content | safe }}
+    </section>
+</article>
+{% endblock content %}