Docs: Use mdbook for docs

Lays out the foundation to better docs.

With this MR, the docs now all live in the docs directory.

In the long term, I would like the docs to be available under the conduit.rs domain. For now, I've gone with the following route:

• master docs: https://conduit-docs.surge.sh
• next docs: https://conduit-docs-next.surge.sh
• MR docs like this: https://conduit-docs-mdbook-docs-preparation.surge.sh

This alleviates the issue of the Markdown docs being from the next branch, while people are encouraged to run the master version.

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Closes #153

• I agree to release my code and all other changes of this MR under the Apache-2.0 license