bug 889536 - mdn sphinx theme cleanup

First, get the sphinx theme:
```
git clone https://github.com/lmorchard/mozilla-mdn-sphinx-theme.git
```
Now re-create the `layout.html` file for the theme:
```
vagrant provision # or manually update settings_local.py to add PRODUCTION_URL
manage.py generate_sphinx_template > layout.html
cp layout.html path/to/mozilla-mdn-sphinx-theme/mdn_theme/mdn/layout.html
```

Then get zamboni so you can build its api docs using the sphinx theme:
 ```
git clone https://github.com/mozilla/zamboni.git
cd zamboni/docs/api
```
Update the `html_theme_path` in `conf.py` to use your local theme with the updated layout.html - e.g.,
```
...
#import mdn_theme
html_theme_path = ['/Users/lcrouch/code/mozilla/mozilla-mdn-sphinx-theme/mdn_theme',]
...
```
Now re-make the API docs with new theme, and open them for testing:
```
make html
open _build/html/index.html
```
Everything mentioned in https://bugzilla.mozilla.org/show_bug.cgi?id=889536#c8 should be fixed.