What's new sections for v4.0 - #9581

The goal of this PR is to add entries to the 'What's new in v4.0?'. I've added section headings for things I think are worth highlighting, but of course we could add/remove if needed. I suggest that we try and write all of these as part of a single PR as it will make it easier to go over after and make sure everything is consistent etc.

So if you would like to write one or more of these sections (or another section that is missing) please either just push a commit directly to my whatsnew-4.0 branch or open a PR to my branch if you prefer or don't have the adequate permissions to push to my branch. If you aren't sure how to do either of these, feel free to ping me on slack.

I'm going to tag and assign people who've been involved in developing the functionality/changes in this page or oversaw the relevant PRs:

• Cosmology: @aconley
• New models: @karllark
• Table: @taldcroft
• Constants and units: @stargaser (draft done by @mhvk)
• Galactocentric frame updates: @adrn
• New ymdhms Time format: @mhvk @taldcroft
• Plotting Time values: @astrofrog
• Parsing high-precision values in Time: @aarchiba (draft by @mhvk)
• Improved compatibility of quantity with numpy ufuncs: @mhvk
• 1-d profile plots and default labeling with WCSAxes: @Cadair
• Function to fit WCS to pairs of pixel/world coordinates: @eteq
• WCS transformations with Time: @astrofrog
• Time series fold() improvements: @astrofrog
• Cache import/export: @aarchiba
• Performance improvements for table: @taldcroft
• Improved handling of leap seconds: @mhvk
• Modeling API changes: @nden
• Uncertainty/Distribution API changes: @mhvk

If you don't have time to work on this over the next few days, please put a message and I'll find someone else who can write it or I can write it myself.

Tips:

• If you are a maintainer, please also check over https://github.com/astropy/astropy/pulls?q=is%3Apr+label%3Awhatsnew-needed+milestone%3Av4.0+is%3Aclosed to see if any other features/improvements for your sub-packages need to be mentioned and feel free to add a new entry.
• To avoid using up too much CI time, please be sure to include [docs only] in your commit messages!
• Don't force push to this branch!

Once most of the sections are here I'll go over and try and make sure everything is consistent.