Update instructions for blog PRs

[fsmunoz]

Reflect how the first PR should be made on the contributor-site.

Which issue(s) this PR fixes:

Fixes #

[fsmunoz]

/cc @kaslin @sftim

[fsmunoz]

I've pushed the version that was on hackmd for the last month or so, and that (for better and for worse), covers the topics raised here.

[jberkus]

Looks good. Thanks for doing this.

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fsmunoz, jberkus

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[46kumarshivam]

assign/