docs for the new discovery selector feature

[jmazzitelli]

part of: kiali/kiali#7546 part of KEP: https://github.com/kiali/kiali/blob/master/design/KEPS/namespace-discovery/proposal.md

server PR: https://github.com/kiali/kiali/pull/7592 operator PR: https://github.com/kiali/kiali-operator/pull/797 helm chart PR: kiali/helm-charts#274 docs PR: https://github.com/kiali/kiali.io/pull/807

Netlify links:

• https://deploy-preview-807--kiali.netlify.app/docs/faq/installation/#operator-fails-due-to-cannot-list-resource-clusterroles-error
• https://deploy-preview-807--kiali.netlify.app/docs/faq/general/#why-isnt-my-namespace-in-the-namespace-selection-dropdown
• https://deploy-preview-807--kiali.netlify.app/docs/installation/installation-guide/example-install/
• https://deploy-preview-807--kiali.netlify.app/docs/configuration/namespace-management/