Docs migration

[franciscoaguirre]

In order to have a central place for XCM, the docs have been migrated to the polkadot wiki.

From now on, we should contribute to the docs in web3 foundation's polkadot wiki repo.

[KiChjang]

GitHub also needs to know how to redirect to the wiki as well.

[KiChjang]

#48 shows a deprecation notice; I'm thinking however that we may want to keep this repository just for the examples here, since the W3F wiki repo doesn't seem to contain them.

[franciscoaguirre]

Yes, we'll keep this repo for the examples, we could rename it to xcm-examples or xcm-cookbook and focus only on that.

[CrackTheCode016]

Definitely should repurpose this to code examples/cookbook/"how to do X on XCM", it will be very useful to link to even in the Wiki along with other external places.