Adding link checker to the CI (closes
https://github.com/paritytech/polkadot-sdk/issues/993). It would be nice
to have the docs people own this and extend accordingly.
Currently all known-bad links are excluded, but we should one-by-one
include those as well until all are fixed.

This check now ensures that 1) no new broken links are introduced into
`.rs` files and 2) that no old links break unnoticed.

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>