Release ink! 3.0.0-rc8 (#1105)

* Bump rc7 to rc8

* Update changelog

* Apply suggestions from code review

Co-authored-by: GreenBaneling | Supercolony <XgreenX9999@gmail.com>

* Fix `polkadot-js/api` links + mention PR #1100

* `s/polkadots/polkadot/g`

* Fix typo: contract ➜ contracts

Co-authored-by: GreenBaneling | Supercolony <XgreenX9999@gmail.com>
Co-authored-by: Hernando Castano <hernando@hcastano.com>