update polkadot, substrate, cumulus readme (#1182)

* update readmes

* temporary ReadMe for the Polkadot SDK

* delete welcome readme

* update links on substrate readme

* update links on polkadot readme

* update links on cumulus readme

* update overseer feature comment

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* Update cumulus/README.md

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* Update cumulus/README.md

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* update gitlab links

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* terminal friendly convention

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>