Add learn page

[hanbyul-here]

Opening to share the preview, and flag the need for the contents.

Closes #252

To do:

• [x] complete content from final version of doc for first release
• [x] hide the newsletter section for now, until we have a first newsletter
• [x] make some indication of what is an upcoming event and what is a past event (doc link)
• [x] add link to footer nav to the "Press Information" section. Title "Press Info"

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[hanbyul-here]

I am going to revert veda-ui to respect the proper UI update in https://github.com/NASA-IMPACT/veda-config-ghg/pull/264. Learn page won't be accessible through the main nav, but it will show up again once it is merged into staging. (The page is also still accessible through url /learn)