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 <[email protected]>

* Update cumulus/README.md

Co-authored-by: Liam Aharon <[email protected]>

* Update cumulus/README.md

Co-authored-by: Liam Aharon <[email protected]>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <[email protected]>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <[email protected]>

* Update polkadot/README.md

Co-authored-by: Liam Aharon <[email protected]>

* update gitlab links

Co-authored-by: Liam Aharon <[email protected]>

* terminal friendly convention

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Co-authored-by: Liam Aharon <[email protected]>