docs: Move user guide to index page

[jpsamaroo]

The index page (docs/src/index.md) is too basic and focused on users who care more about the DTable internals, rather than how to use it. I propose moving either the User Guide or the quickstart examples (proposed in #37) to this index page, and moving the content that's currently in the index page to a "Background" or "Technical Details" page or something like that.