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]

Hi @wendy-ha18. Thanks for your PR.

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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!

[jberkus]

This is mostly pretty good, just a few changes.

[jberkus]

Looks good!

[k8s-ci-robot]

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