XCM Cookbook (#2633)

# Context

XCM docs are currently an md book hosted with github pages:
https://paritytech.github.io/xcm-docs/.
While that's fine, it's not in line with the work being done in the
polkadot-sdk docs.

# Main addition

This PR aims to fix that by bringing the docs back to this repo.
This does not have all the information currently present in the mdbook
xcm-docs but aims to be a good chunk of it and fully replace it over
time.

I also added the sections `guides` and `cookbook` which will be very
useful for users wanting to get into XCM.
For now I only added one example to the cookbook, but have ideas for
guides and more examples.
Having this docs be in rust docs is very useful for the cookbook.

# TODO

- [x] Use `FungibleAdapter`
- [x] Improve and relocate mock message queue
- [x] Fix license issue. Why does docs/sdk/ not have this problem? (Just
added the licenses)

# Next steps

- More examples in the cookbook
- End-to-end XCM guide with zombienet testing

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Co-authored-by: command-bot <>
Co-authored-by: joe petrowski <[email protected]>