Move the XCM spec and RFC process under the Polkadot Fellowship

[acatangiu]

Since XCM is under the responsibilities of the Polkadot Fellowship, the XCM format repo should me moved under the polkadot-fellows GH org.

Currently the xcm-format repo is made up of:

1. XCM (latest) specification
2. RFC proposals for protocol/spec changes

(2) can be done using polkadot-fellows/RFCs,
but (1) doesn't really fit in here, so I propose keeping it under its own repo.

[acatangiu]

@xlc @andresilva @ggwpez @bkchr who has write access on the fellows org and can help with creating a new repo?

[xlc]

Best to use the transfer ownership feature to keep the PR and issues

[acatangiu]

The repo has been migrated: https://github.com/polkadot-fellows/xcm-format

Now we need to migrate open XCM RFCs to this repo.

[acatangiu]

cc @franciscoaguirre