What's New 0.4 coordinates - take 2

Add on to PR #2676 from @eteq:

Update coordinates What's New to emphasize positive new features.
   
- Moves most of the original content into a new section "Migrating
  from pre-v0.4 coordinates" which is in the main coordinates index page.
- Adds a sphinx note up top in the index page pointing to the possible need for
  migrating code with a link to the new section.

typo: convenience