docs: add a basic guide to the tech stack

[ojeytonwilliams]

The hope is that this should help with onboarding new contributors as well as reminding longer standing ones. One of the biggest barriers I had to contributing was figuring out what the various parts of the tech stack were responsible for and how they interacted.

This should help people get up to speed quickly so they can actually make stuff.

If #685 goes in, the directories will need updating to reflect those changes.

[gitpod-io]

[ojeytonwilliams]

@allella let me know if this reads okay and you think it's a worthwhile addition. I was thinking it would help, particularly since I've highlighted some beginner friendly issues with #691

[allella]

@ojeytonwilliams I think this is a fine addition. I may tweak some of the content a bit, but nothing to hold up merging it in.

My main question is if we should consolidate the Tech Stack section from README and/or link to this new section of CONTRIBUTING.md.

I don't think we want to maintain the exact same content in two places since it's likely we'll forget to update both. Though, having a very high-level set of technology bullet points, like we have in the README, and then linking from that section directly to this more verbose section of the CONTRIBUTING.md could be a middle-ground that mostly avoids duplication but still gives a highlight in the README.

Thoughts?

[ojeytonwilliams]

@allella I think this is fine, so I'm merging, but if I missed something just let me know and I'll fix it in another PR.