Add a route to serve guidance pages

Rather than hard coding the page titles, let’s just accept anythin
 that’s a real template in the guidance folder – will make it easier for
 Karl to edit and create pages.

app/main/views/index.py

@@ -7,6 +7,7 @@ from flask import (
     url_for,
 )
 from flask_login import current_user
+from jinja2.exceptions import TemplateNotFound
 from notifications_utils.international_billing_rates import (
     INTERNATIONAL_BILLING_RATES,
 )
@@ -325,6 +326,17 @@ def trial_mode_new():
     )
 
 
+@main.route('/guidance')
+@main.route('/guidance/<slug>')
+def guidance(slug='index'):
+    try:
+        return render_template(
+            'views/guidance/{}.html'.format(slug)
+        )
+    except TemplateNotFound:
+        abort(404)
+
+
 # --- Redirects --- #
 
 @main.route('/roadmap', endpoint='old_roadmap')