Relay-parent digest logs for parachains (#2552)

* add digest item for relay-parent to primitives

* add a relay-parent-storage-root digest as a workaround

* more docs

* deposit log in pallet-parachain-system

* even more docs

* fix duplicate imports after botched mertge

* fix hyperlinks in docs

* clean up match

Co-authored-by: Bastian Köcher <git@kchr.de>

* improve docs

* fix typo

* add number to the digest item

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Co-authored-by: Bastian Köcher <git@kchr.de>