DOC: Added call out to API guidelines to contribute + small API guidelines reorg

[story645]

PR summary

Spun docs changes out of #27641 to let that one go through faster. Changes here are:

• based on @timhoffm's suggestions at MNT: fix api changes link in PR template #27641 (comment)
  • changed API->changes to API->guidelines
  • prioritized new features
• reworked heirarchy in API Guidelines so top level is announce, add new features, deprecate
• added a call out in coding guidelines to direct to API guidelines:

• and since the white space balance feels weird to me, commit 2:
  

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[timhoffm]

Take or leave: Announcing is done after one has done the implementation. Therefore I'm slightly in favor of moving this section to the bottom so that the new order is:

• Add new API and features
• Deprecate API
• Announce new features, API changes, and deprecations

[story645]

Agree, I think what I've done here is blend overview with the announcement specific relevant stuff, so I may separate that out.

[story645]

@timhoffm updated it so that the announce section is a bit less overview and pushed it to the bottom.