feat: Add product roadmap

[sarahmonster]

This adds a roadmap so users are aware of what's "on our list":

It could probably be made prettier in the future, but let's get the content sorted first and then I can work to finesse it later.

I've also included a few other wee changes:

• Updates the charitable donations section for 2021, only 11 days later.
• Reframes the help section as an FAQ. (I also have a bunch of documentation to write to flesh this out, so I can handle it as a separate PR if that would be easier to review & approve!)
• Tests out a small change to the email signup form (basically adding dual tags isn't working right, but I'm struggling to test it locally).

Closes #77.

[tofumatt]

🥳

I dig it! Might be good to replace the COVID-specific charity image eventually, and I didn't test your newsletter thing, but I love this.

🚢