[Docs] Clean up API references and restructure docs

[ArjunBhuptani]

Now that 7.0 is out, it's worthwhile investing some time in our docs.

There's a few things we should do:

1. Restructure docs so that the guides, quick start, etc. are categorized by the actual target user. E.g. Wallet docs, Browser app docs, node docs.
2. Update our positioning in intro sections + update FAQ
3. Add more guides for things like starting node locally, configuring node, different transfer types, etc.