Add devhub crate

[sacha-l]

This adds the devhub crate for hosting Polkadot SDK reference docs and tutorials, following the structure proposed here.

The reference docs module currently contains extrinsics.rs and weights.rs with a "This page is currently under construction." message for now. I mostly created this so we can start collaborating with experts to help craft these ref docs in separate PRs. Kian created this list of potential reference material docs that would live here too. The tutorial module would be where I envision the master tutorial will live.

With this, individual ref docs will have their own modules like this:

[kianenigma]

Should move docs to document too. As for the rest of the crates, I would leave it all empty.

[kianenigma]

see https://github.com/paritytech/polkadot-sdk/compare/kiz-frame-api...kiz-umbrella-crates?expand=1