docs: add setup guide for OpenRouter

[sambarnes]

Description

gm :wave: I recently started using the continue plugin on Jetbrains & love it so far!

started some documentation to help smooth out the onboarding experience with OpenRouter specifically -- happy to iterate on it if y'all think it needs adjusting :bowing_woman:

Checklist

• [x] The base branch of this PR is preview, rather than main