Group pages within their own directory

Add `pages` as it's own content type, and move all page content into
the appropriate directory.

Any references to `layout` in the YAML front matter has also been
removed as it's implied by the content type. Except for `about.md`
which does explicity use a different layout.

source/_pages/test-driven-drupal.md

@@ -0,0 +1,49 @@
+---
+title: 'Test Driven Drupal: The Book'
+mailchimp_url: 'https://oliverdavi.us18.list-manage.com/subscribe/post?u=b4ac8dd177796d37b93f9c285&id=033c84e0d5'
+contact_email: 'oliver@testdrivendrupal.com'
+---
+{% block content %}
+<div class="markup spaced-y-4 mb-6" markdown="1">
+Having [given talks][1] and workshops, been a guest on podcasts and [written articles][0] about automated testing in Drupal, I’m currently in the planning phase of a book and potentially some accompanying screencasts about it, focussing on Drupal 8.
+
+I’m still thinking about what use-cases to cover and examples to include, but here are some of the things I’m considering:
+
+- What things to test, and what not to test
+- The different types of available tests, and when to use each
+- How to write testable code
+- What happens when I run a test?
+- How to run tests in the Drupal UI
+- How to run tests with the `run-tests.sh` script
+- How to install, configure and run tests with PHPUnit in Drupal 8
+- Viewing HTML from run tests
+- How to write your first test
+- Debugging tests
+- How to organise your test files
+- Selecting the right base class and using test traits
+- Writing your own base test classes, traits and assertions
+- Managing dependencies for your tests (fields, configuration)
+- Creating users, checking access with roles and permissions
+- Creating pages and blocks with Views and testing the output
+- Creating pages with routes and controllers and testing the output
+- Testing custom plugins
+- Testing queuing items and processing queues
+- Testing sending emails
+- Testing custom Twig filters and functions
+<!-- - Testing data migrations -->
+<!-- - Building and testing APIs using RESTful web services module -->
+- Running tests as part of your continuous integration pipeline
+
+I’ll most likely be publishing it via Leanpub, and will be sending free chapters, early-bird discounts and links to screencasts and blog posts as I write the book to subscribers of the mailing list.
+
+If you have questions or would like to suggest something for me to include in the book, please <a href="mailto:{{ page.contact_email }}">contact me</a>.
+
+**Enter your email address to subscribe to the Test Driven Drupal mailing list and be notified of any updates.**
+</div>
+
+{% include 'pages/book/signup-form' %}
+{% endblock %}
+
+[0]: /articles/tags/testing
+[1]: /talks/tdd-test-driven-drupal
+[2]: /contact