Update _index.en.md for manifest installation

[dk03051996]

This pull request is being raised to update docs if anyone if trying to install using manifest. This issue was raised on our slack channel. Below are the few updates that this pull request will do

• removed this command "kubectl create namespace fission" because the same is being used below and add a line for users to use fission as default namespace
• added this command "kubectl config set-context --current --namespace=default " to change context to default namespace after installation because if user does not change namespace and try to create env for node/python etc then pods for pool manager will not come up as default namespace is set to default for all fission.
• updated the same command for basic, minikube/kind and openshift.

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