Release 1.4 roadmap

[mperrin]

The following items were on the Release 1.3 roadmap (#808) but were deferred beyond that release:

• [ ] NIRISS GR700XD model improvements. (not yet a PR) @obi-wan76
• [ ] add some smaller issues regarding improvements in a couple of function @obi-wan76
• [x] Documentation about PSF subtraction examples @obi-wan76
• [ ] Notebook example on how to create very large PSFs @obi-wan76

Draft PRs intentionally delayed to after 1.3:

• [ ] #675
• [ ] #838