Better layout templates

Now that we've got a few different layouts under our belt it's time to make them more reusable. This PR allows us to start a new 2-1 layout as simply as this:

```jinja
{% extends "layout-2-1.html" %}

{% block content_main %}
    <h1>Main content</h1>
{% endblock %}

{% block content_sidebar -%}
    <p>Sidebar</p>
{%- endblock %}
```

Want one with a bleedbar?

```jinja
{% extends "layout-2-1-bleedbar.html" %}
```

Let's leave this open for a little bit while I tackle a few things:

- [ ] document layout chain better in the layouts section (started in on this on the content-base.html section)
-  [X] move layouts section to docs, probably rename to avoid conflict with Less layout docs
- [ ] rename `.content_block` to just block?
- [X] rename base layout .html files so visually it appears there are only a few layouts to choose
- [X] rename section_vars to section
- [ ] remove scoped from layout blocks? (not exactly sure if this is needed but i added it to all, would be nice to remove them if not needed)
- [ ] rethink heavy use of imports since they are cached. for example if you have a file with the same name in two directories and you import them into two different templates the first template that gets accessed will cache the import relative to it. Then if you hit the other directory it won't grab that import, it will grab the cached import. boooo.