Add below-the-fold doc categories on docs main page

[oubiwann]

Like what the OpenStack docs do:
- http://docs.openstack.org/

We can sketch this out and then fine-tune when the IA tickets are done (see lfe/docs#49).

[oubiwann]

Here are the main sections identified in lfe/docs#49:

1. Learning LFE
2. Reference
3. Building Applications
4. Tooling
5. Community Libraries
6. Contributing
7. Community Resources
8. Other Media
9. History

Maybe these might be better:

1. Learning LFE
2. Reference
3. Building Applications
4. Tooling
5. LFE Libraries
6. Getting Involved
7. Community Resources
8. History

With the "other media" items getting moved into the other sections, as appropriate.

[oubiwann]

I may add sub-categories to the mockup, but I'm counting this done now.