Revise https://etcd.io/docs/v3.5/tutorials/how-to-create-locks/

This one requires @wenjiaswe or other members of etcd as there's a bit of missing explanation I can't find anywhere else in the docs :)

This issue was created in preparation for the Contributor Summit docs sprint.

File: https://github.com/etcd-io/website/blob/main/content/en/docs/v3.6/tutorials/how-to-create-locks.md

Per https://github.com/etcd-io/website/issues/765, do the following:

- Add prerequisites (etcdctl and a currently running etcd cluster are required)
- Define all variables and flags used in the examples in a bulleted list (in this case, `--endpoints, `locks)
- Explain in a bit more detail why you might want to create a lock (i.e., what kind of client needs this?) -- this is the part you'll need an etcd maintainer for
- Maybe: Add a section on how to release a lock, as there does not appear to currently have one and I suspect you need one ;)

When done, the document should have the following sub-sections (## H2 headings):

- Pre-requisites
- Creating a lock
- Releasing a lock (**new section**)