Migration: v4 to v5 migration guide

Author the user-facing v4 to v5 migration guide: install changes,
command map, behavior diffs, known gaps.

## Definition of Done

- [ ] Migration guide published
- [ ] v4 to v5 command map (companion to in-repo docs)
- [ ] Behavior diffs catalog
- [ ] Known gaps page
- [ ] Linked from `func --help` footer + Learn landing page

Start by reviewing the companion in-repo docs and running `func --help` to understand the current commands and footer. Document install changes, the v4-to-v5 command map, behavior differences, and known gaps, then link the published guide from the `func --help` footer and Learn landing page.

Written by the indexing model from the issue text.