docs: new code intel docs index page

Created by: mrnugget

I've changed the structure of the start page of our docs to answer the following questions above the fold:

• what does code intelligence give me
• what does code intelligence consist of
• how can I start using it
• what do I need to do to use which feature
• where can I use code intelligence

As always: this is the first iteration. I have some more ideas on how to tweak this and improve it a bit more, but I think we should really start answering these questions at the start of our docs, especially since we link to our docs from the product.

Test plan

• sg run docsite and look at it locally