fix: improved membership request template

[dandawg]

This PR improves the instructions on the membership request template, fixing in particular the formatting and instructions on submitting a PR.

The issue was that the flow for becoming a Kubeflow member (as outlined here) call for creating an issue. The issue template then mentions "Please test your PR". This is confusing because you haven't created a PR yet at this point.

In addition, there are no easily findable instructions stating that you need to "modify the org.yaml in the github-orgs/kubeflow directory. I had to search around the source code to find them when I was becoming a member.

This PR makes the process easier by formatting the membership request template, and by including more detailed instructions on what is required in the PR.

[google-oss-prow]

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[google-oss-prow]

Hi @dandawg. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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