Initial documentation restructuring.

[benwaldie]

This is an initial pass at restructuring the Swift Package Manager documentation. Additional review, content changes, and restructuring pending.

[ddunbar]

This looks awesome, thank you!

[ddunbar]

I'll do a more detailed review at some point, but this already looks like it is a major improvement on our existing structure. I'll see if anyone else has an opinion, but I would lean towards merging this ASAP and continuing to iterate on it from there.

[aciidgh]

I agree, this look great direction

[abertelrud]

Nice! I agree, the structure improvements are the most important thing to land as soon as possible, and then we can do the rest with incremental changes. Thanks, Ben!