Docs: Split migration guide into Migration Guide + What's new in 1.0 #2527

Proposal: Split migration guide into two user-focused documents

Context: I think the current migration guide is too long and reads more like developer-facing documentation. For the 1.0 release, I’d like to propose splitting it into two documents with different purposes.

What I’m proposing:

1. Migration guide (short, “need to know”)

   • Audience: Anyone upgrading from pre-1.0 who needs to fix breakage.
   • Content: Breaking changes only; minimal before/after code; “what must I change?”
   • Length: Short enough to read in a few minutes, with links to the second doc for detail.

2. “What’s new in ArviZ 1.0?” (informative)

   • Audience: Users who want to understand what changed and how to use new features.
   • Content: Full narrative (DataTree, rcParams, stats, plots, etc.) with context, rationale, and examples—still user-focused, not internal/developer detail.
   • Length: Can be longer; this is the “full story” for users.

Why: One doc stays actionable for people in a hurry; the other serves everyone who wants to read about 1.0 without wading through implementation details.

What I’d like to do: I’m prepared to make these changes and open a that would add “What’s new in 1.0?”, trim the migration guide to need-to-know content, cross-link the two, and update the docs index. I’d welcome feedback on the approach before or during the PR.