Generate index page

[SeanTAllen]

We currently build docs but do not generate an index page.

https://github.com/ponylang/main.actor/blob/master/docs/index.html should be replaced (and the logo can be deleted unless reused as part of a nice design) with an alphabetical listing of all libraries. For each library, each version should be listed and linked to in reverse order from "highest" to "lowest" in terms of version.

Valid versions are in the form of x.y.z or x.y where x,y,z are digits.

As an example, if a library had versions 1.0.0, 0.2.0, 0.1.0, 1.1.0, 0.1.1 then the order should be:

• 1.1.0
• 1.0.0
• 0.2.0
• 0.1.1
• 0.1.0

[SeanTAllen]

The initial gatsby website that @ChristopherBiscardi put together has this covered.