Add overarching "how to get involved" page #148

@ProjectPythia/outreach suggests we create an overarching space on the main portal highlighting How to get involved. The initial ways we expect community involvement we picture are

• Technical contributions: code, notebooks, infrastructure, etc.
• Feedback: using and reviewing our materials for educational feedback
• Ambassadorship: sharing us! social media, in-class, at-conference, etc.

where we'd have each of these described briefly on this page and then direct people to relevant pages for elaboration, e.g. to the contributor's guide.

We then could boil this page down to a flyer for sharing over social media or in-person as necessary, with a link/QR code to "how to get involved".

We plan to bring this up at Thursday's meeting for feedback and moving forward!