Add a document describing when to use ciso8601

[movermeyer]

What are you trying to accomplish?

cPython 3.11's datetime.fromisoformat method is finally feature-full enough to be worthy of comparison with ciso8601, and indeed at present outperforms ciso8601 for timestamps without timezones (Until #130 gets merged).

Finally, many Python users have no need for ciso8601 at all! 🎉

We're going to need some writing to let people understand the cases in which they might not need ciso8601.

What approach did you choose and why?

I wrote a new document that outlines the relevant considerations when choosing a ISO 8601 timestamp parser.

I used GitHub's support for Mermaid diagrams to produce a flow chart. That was really pleasant; I found Mermaid to be delightful and intuitive.

The diagram will become much simpler if I:

• Add the Python 3.11 changes movermeyer/backports.datetime_fromisoformat#21
• Add support for week dates and ordinal dates #131

As there would be fewer decision points, and fewer reasons to not use ciso8601.

What should reviewers focus on?

Copy editing.

The impact of these changes

Hopefully, users will become aware that they might not need ciso8601 at all, and can use the built-in datetime.fromisoformat method in many cases.