Whats new page for 3.1

[story645]

PR Summary

Put all the what's new entries together into something hopefully vaguely resembling cohesion based on the directions here. Was a bit confused as documented in #13853

Also having trouble rendering the toc properly.
(ETA: I'll do the API changes compilation too if this one was mostly on target)

[tacaswell]

@story645 Added some commits, rebased, and force-pushed.

[tacaswell]

Want to tweak this to use include and ifconfig like the API changes PR did to take this simpler going forward, but very 💤 so stopping for the night.

[story645]

@tacaswell the include ifconfig stuff is I think what I was confused about in #13853

[tacaswell]

If I understand ifconfig correctly (and used it correctly), then when we tag and build the docs it should automatically exclude the glob tocs.

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.1.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 26e4f4a25367232192a0c21a50635877ee2a3ac4

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #13856: Whats new page for 3.1 '

4. Push to a named branch :

git push YOURFORK v3.1.x:auto-backport-of-pr-13856-on-v3.1.x

5. Create a PR against branch v3.1.x, I would have named this PR:

"Backport PR #13856 on branch v3.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[tacaswell]

backported and directly pushed in b1a6fba