Move doc module to mdbook?

[LordMZTE]

Hey, I've seen that sled seems to have a lot of documentation which isn't directly related to rust code in the doc module. I think that's pretty unusual, and I would find it easier to read through, and better organized if it were seperate from the rustdoc.

The solution that many rust developers use for this is mdbook. Many big rust libraries (such as serde or druid) use mdbook for documentation, best practices and guides as well, what about doing so too? The current content of the doc module should also be pretty easy to tranfer to mdbook, as theyre both markdown.