Add high-level feature list page to the website

[jheretic]

Due to Commotion's historical nature as a multi-platform, fairly disaggregated project with a large number of component parts, we haven't previously had a high-level feature list page like many other open-source projects. This would be a good thing to add to the About menu.

[andygunn]

@jheretic I can take issue #300 if you take this one. Assigning you.