Remove codespaces and add link to k3d config

[Reshrahim]

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Description

This PR removed Codespaces as it is disabled for the project. We are working with the CNCF team to get it re-enabled. Also ads link to the k3d cluster config

Issue reference

Fixes: https://github.com/radius-project/docs/issues/1171

[brooke-hamilton]

Having the k3d cluster config link is great. 🎸