Update to SDV Contribution Day website

Please update the event website to make the following changes:

1. Remove the "Register Now" button in the Jumbotron

2. Remove the "Information for Onsite Attendees" button in the first section.

3. Add a section above the "About the Event" section with this content:

Headline:

Thank you!

Text:

The first SDV Contribution Day has ended. Thank you to our speakers, attendees, and working group members. Presentation recordings will be uploaded to the Eclipse Foundation YouTube channel soon.