Update project level docs (#1734)

* updated project level document

* updated high level overview document

* GRANDPA finality relay sequence diagram

* Parachains Finality Relay Sequence Diagram

* Messages Relay Sequence Diagram

* Complex Relayer Sequence Diagram

* small fix

* Polkadot <> Kusama bridge flowchart

* remove obsolete files

* started polkadot-kusama-bridge-overview.md

* continue polkadot-kusama-bridge-overview.md

* couple more sections in polkadot-kusama-bridge-overview.md

* continue polkadot-kusama-bridge-overview.md

* renew deployments readme

* fixed review suggestions

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* removed obsolete section

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* typo

* Update docs/polkadot-kusama-bridge-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/high-level-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/polkadot-kusama-bridge-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/polkadot-kusama-bridge-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/polkadot-kusama-bridge-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update docs/polkadot-kusama-bridge-overview.md

Co-authored-by: Adrian Catangiu <adrian@parity.io>

Co-authored-by: Adrian Catangiu <adrian@parity.io>