Revamp Overview + Getting Started sections

[valentinewallace]

I'm not sure that what's currently in these sections makes sense. Seems like Overview should be more of an overview of LDK or maybe an index of the docs? Maybe take inspiration from Firebase Overview section or Stripe Overview.

And seems like Getting Started is more of an Architecture section, so should be renamed. If Overview becomes an index/entrypoint to the docs, then maybe Getting Started is unnecessary.

In any case, these two sections need some rethinking.

[valentinewallace]

Closed by #18.