Update Templates Readme - Github Repo links (#5381)

When someone downloads the Polkadot SDK repo and navigates to the
templates folder, the Readme instructions do not work. There is a
getting started script of the Polkadot SDK readme which can be
overlooked (and also it covers only minimal template and not the
parachain/solochain templates).

The instructions of the Readme files are updated such that they work for
anyone on
https://github.com/paritytech/polkadot-sdk
https://github.com/paritytech/polkadot-sdk-minimal-template
https://github.com/paritytech/polkadot-sdk-parachain-template
https://github.com/paritytech/polkadot-sdk-solochain-template

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>