Add banner for JupyterCon 2025

[choldgraf]

This adds a banner for JupyterCon 2025. It re-uses the background from the jupytercon.com banner and it basically re-adds the banner HTML/CSS that we removed in github.com//pull/729 . cc @rpwagner to do with this as he wishes!

Here's what the banner looks like on a 1080 pixel width screen

[rpwagner]

Yes! Thank you @choldgraf! This is more than good enough for me.

[rpwagner]

Adds a banner and the div tag and css looks reasonable.

[krassowski]

It would be nice to make sure the tests pass.

[choldgraf]

It seemed like the two failing links were not related to this PR and also were valid links so I wasn't sure what to do about it. I don't understand the link checking infrastructure so I might be reading it wrong.

https://github.com/jupyter/jupyter.github.io/actions/runs/14348121869/job/40221705591#step:4:858

[krassowski]

so I wasn't sure what to do about it

I would usually open an issue (like #762) and try to fix it later. Or one could count on volunteers who were keeping this CI up and clean to do that later.

But these were very much introduced by this PR:

"file:/github/workspace/_site/index.html":113.101-113.108: error: The element "button" must not appear as a descendant of the "a" element.
"file:/github/workspace/_site/index.html":114.113-114.120: error: The element "button" must not appear as a descendant of the "a" element.

[choldgraf]

Oh yeah - I was just looking at the link checks.

Sure I will open an issue if it is important that buttons are not descendants of links.

[choldgraf]

I've got a follow up PR here:

• Fix CI by removing <a> and <button> nesting as well as ignoring some working links #789

I also fixed the failing links.

Apparently our CI imposes a rule that we cannot nest <a> and <button> inside of each other in either order.