Revise website tutorial how-to-transactional-write

[wendy-ha18]

This PR uses to address this issue: #801 for updating the Guide to making transactional writes.

• Add prerequisites (etcdctl and a currently running etcd cluster are required)
• Define all variables and flags used in the examples in a bulleted list
• Explain txn and why you might want to transactionally write things (might need a maintainer to help with this!) (completed partially, need to ask for advice).

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @wendy-ha18. Thanks for your PR.

I'm waiting for a etcd-io 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.

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[wendy-ha18]

One requirement of issue: #801 mentioned about why you might want to transactionally write things (might need a maintainer to help with this!).

After consideration, IMHO, the main reason we want to transactionally write things are to maintain data consistency and ensure expected state fefore writing, which are similar with this part. Just want to ask maintainer @jmhbnz do you have any opinions on it?.

Do we still need to including this explaination to make the tutorial clearer?

[wendy-ha18]

cc @ronaldngounou @jberkus @nate-double-u for reviewing.

[jberkus]

/ok-to-test

[jberkus]

@wendy-ha18 see linting errors per the test.

[wendy-ha18]

Hi @jberkus I have fixed all of lint test fails and backport v3.6 to v3.5 as well.
Could you plz help me review it again? Thanks a lot Josh!