Review and update Workspace doc

[lread]

Reword:

1. Change "addicted" to "enamored". I like the intent of "addicted" but I think we can use something as powerful but more sensitive.

Clarifications:

1. mention examples assume reader is using poly installed as stand-alone
2. mention a git repo is, by default, created
3. mention that examples commit the created workspace
4. move branch: example to a tip, the reader will be less likely to think they also need to run this variation
5. add trailing forward slash to directories. It makes it easier to see that they are directories
6. add note that user will need to commit the generated workspace files themselves when creating from an existing git repo
7. mention what deps.edn aliases are used for.

Content:

1. Update generated deps.edn :poly alias to use :mvn/version
2. Remove example and text around fetching :latest-sha

[lread]

Ok, @tengstrand, all review feedback has been addressed!

I don't want to put more in this PR so will deal with describing how to bootstrap using poly as a dep separately.