[DO NOT MERGE] Add temporary banner at the top of page

[slesaad]

@deborahUAH requested a temporary banner to advertise AMS workshop on the GHG center homepage. I tried to create something. While it does the job, it is not the best. Can someone from the frontend team quickly clean this out so we can push it out now while we work on a more permanent and better solution for later (we also want to do it for the Summer School)? This one is only gonna stay there for 4 days (till Sunday when the workshop is).

cc @j08lue @hanbyul-here @sandrahoang686

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[faustoperez]

Hi @slesaad @sandrahoang686 @hanbyul-here just a couple of minor tweaks:

• Make the banner 100% width
• Center the text in the viewport on desktop
• Align the text left on mobile,
• Make it push the content below
• I included the mobile styles

Link to Figma file 👉 https://www.figma.com/file/9INQauBWhiRxvOWDGhRrxO/US-GHG-Center?type=design&node-id=663%3A358&mode=design&t=TVyFrkYdt5O9Oor6-1