Changes to support SDK Build

Add icon for external links (will also appear on site build)
Other changes
In [theory](http://sphinx-doc.org/invocation.html) it should be possible to pass these values into sphinx on the command line using something like:

```
make html SPHINXOPTS="-D release='6.40'"
```

However I can't get it to work on Windows (not tested on Linux). Note that this is also why I can't get the SDK build to be a command line invocation. Jukka might have more luck at working out why.