Add more overview info on docs front page

[jasongrout]

This is probably still a bit wordy, but it's nice to have some high-level context on the front page.

Also I deleted the developer_docs page which seemed largely empty.

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/introdocs

[ibdafna]

LGTM

[jasongrout]

Awesome, thanks for the review. We can keep iterating to make the docs more approachable.

[jasongrout]

@meeseeksdev please backport to 7.x

[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 7.x
git pull

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

git cherry-pick -x -m1 106362178c1e2a84e38ecd6d6d3b43f923c1c508

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

git commit -am 'Backport PR #3488: Add more overview info on docs front page'

4. Push to a named branch:

git push YOURFORK 7.x:auto-backport-of-pr-3488-on-7.x

5. Create a PR against branch 7.x, I would have named this PR:

"Backport PR #3488 on branch 7.x (Add more overview info on docs front page)"

And apply the correct labels and milestones.

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

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

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