Add Nextjs assessment doc

[echappen]

Changes proposed in this pull request:

Closes #17

I've ported my rough notes on NextJS from our Engineering doc into the repo docs. I expect these notes to serve as the basis for an upcoming ADR for #28.

Looking for feedback from @hursey013 @sknep and @jduss4 on:

• What is unclear or confusing?
• Is there info I should've included (or something I should've prototyped) that I missed?

I feel like the Accessibility, Security, and Telemetry elements are the least thorough and provide more theoretical information than prototyped solutions. Testing, Routing, API handling are the most thorough.

@jduss4 If you want to fill out any notes for the Deployment section prior to merging this, please do.

Security considerations

None - documentation only

[sknep]

@apburnes no pressure, just thought you might like to eyeball this

[jduss4]

I'm really impressed by everything you've been testing out. It's very thorough and easily understandable! Thanks!

[hursey013]

This is really great work! Being able to get the entire team up to speed on the capabilities and details of Next.js before we begin building is so valuable - thank you!