Updates conference details on event page

[exoticDFT]

Description

• Updates past conference to grey background to signify USRSE'25 being over on conference event page
• Puts in some placeholder information for USRSE'26 conference on main events page

Checklist:

• I have previewed changes locally or with CircleCI (runs when PR is created)
• I have completed any content reviews, such as getting input from relevant working groups. If no, please note this and wait to post the PR to the #website channel until the content has been settled.

[exoticDFT]

Previews at:
https://output.circle-artifacts.com/output/job/f5be9b64-8747-4e9b-840d-04deea6d54bc/artifacts/0/usrse.github.io/events/index.html
https://output.circle-artifacts.com/output/job/f5be9b64-8747-4e9b-840d-04deea6d54bc/artifacts/0/usrse.github.io/events/conference/index.html
I did not add the USRSE'26 event to the second page yet, since the details would still be incomplete.

[cosden]

I made a suggestion to add a little more detail, but it's not necessary! Looks good, much better than leaving USRSE'25, and can always be updated as more details are available.