PEP 12 & 1: Move Specification before Rationale

[dstufft]

Having the Rationale come before the Specification makes PEP documents more awkward to read and write as the PEP is expected to provide a rationale for the decisions made in the specification, before those decisions have even been introduced. This leads to either having to have the rationale play double duty for both introducing the decisions and explaining why or requires skipping to the Specification section, then coming back up to the Rationale.

With this change, a PEP flows in a more natural order of:

1. Elevator Pitch
2. Why should things change?
3. What are you proposing.
4. Why are you proposing that?

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📚 Documentation preview 📚: https://pep-previews--4802.org.readthedocs.build/

[dstufft]

I just happened to mention this annoyance I've had with PEPs to @warsaw and he recommended I make a PR to change it, so here I am :)

I'm keeping this as a draft until #4801 so I can use the fancy new Change History section,

[warsaw]

My other PEP 12 changes have landed. Please merge this one and then I will post to DPO and update the Post-History and add a Change History section. Progress!