docs: rewrite `show` section as two separate modes for clarity

[elliot-100]

I feel that from a user's POV, poetry show has two fairly separate modes, 'list packages' and 'single package info'.

• This PR separates them in documentation, which I feel is clearer.

  For example it shows that 'single package info' only has one useful option, --tree --why.

  (I think the code should probably be refactored to better separate the cases and drop a few interactions between but that's one for another day.)

• It adds missing example for the `list packages' mode.

• It explains what 'poetry show' actually tells you, resolving my very old discussion question #7311.

• It also improves ordering of options and wording.

• It does not attempt to cover all the colour coding that may appear in show output. To be honest it's inacesssible design to use colour alone to convey information.

[TODO: check CLI --help text and error messages]

Pull Request Check List

Resolves: #7311

• [ ] Added tests for changed code -- not relevant for docs-only change
• [ ] Updated documentation for changed code -- not relevant for docs-only change