Add 0.4 what's new section about coordinates

Also adds a  doc ref that was needed in docs/coordinates/representations.rst
Typo fixes from @mdboom's suggestions
added missing space

[ci skip]
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" in the main coordinates index page.
- Adds a sphinx note up top pointing to the possible need for
  migrating code with a link to the new section.
Fix docs typo
Merge pull request #2680 from taldcroft/whats-new-04-coordinates2

What's New 0.4 coordinates - take 2