PEP 1: Update to reflect best practices in PEP annoucement/discussion

[CAM-Gerlach]

This updates PEP 1 (and a few small relevant parts of PEP 12 and the template) to reflect present real-world practice by the SC , as well as the recommendations we agreed on in #2266 , in order to clarify the current process for official discussion, announcement and resolution of PEPs, and improve the experience for PEP authors, the SC/delegates and the community, and avoid confusion, uncertainty and mistakes around these points, and update/remove related out of date content that no longer accurately reflects the de jure or de facto process.

As part of this change, it re-organizes the various scattered, sometimes duplicative or even contradictory paragraphs discussing PEP discussion, announcement and similar into one coherent section, with a clearer, more sequential and logical ordering of the overall process, and adds additional clarifying detail and guidance on points that were previously considered implicit or not referred to at all.

To note, I did not make the changes proposed for the Post-History field (beyond those intrinsically linked here), leaving that for a separate PR (which shouldn't require SC review, as those changes are purely technical in nature). Additionally, while incidental to updating relevant outdated guidance on this specific topic, I did clarify the time and process to submit a PEP for SC consideration (another major point of confusion for some of the recent PEP author situations that motivated this change), further clarification and updates on that will be handled separately, with appropriate SC review. I also noticed some smaller, unrelated items that were out of date, that will also be addressed separately.

Following editorial review by the PEP editors and those involved in that discussion, I'm thinking we'll want to request SC review on their issue tracker before we merge this?

Fixes #2266