Add help page, reroute help to documentation

Created by: christinaforney

We would like to surface more information to our users who are having trouble finding information. This change introduces a help page that has information for them to figure out the best place to go for help. It also includes a new link to a product feedback form in Hubspot. For now, this form will just be an external url and open a new tab, we will eventually build this into the product in a more thorough way (like the NPS forms).

This change also moves the functionality that used to occur with the /help path and changes this to /documentation which is a more accurate representation of where you will end up.

I would love feedback on the copy/messaging. One idea I was considering was making the primary action for each section to be a secondary button (like the form submission).

Question for the @sourcegraph/web team - I couldn't figure out when we're supposed to use a <Link> object vs. an a tag <a href=""> for the links. Let me know what is best practice so I can update consistently!