[Docs] Add docs about adding a new app to SSR router

[damassi]

This adds some docs around adding a new app / route to force, via our SSR router.

[ArtsyOpenSource]

	Fails
🚫	Danger failed to run dangerfile.ts.

Error RangeError

Maximum call stack size exceeded
RangeError: Maximum call stack size exceeded
    at Object.parseExprAtom (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:3600:16)
    at Object.parseExprSubscripts (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8483:23)
    at Object.parseMaybeUnary (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8463:21)
    at Object.parseExprOps (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8329:23)
    at Object.parseMaybeConditional (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8302:23)
    at Object.parseMaybeAssign (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8249:21)
    at Object.parseMaybeAssign (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:2672:18)
    at Object.parseExprListItem (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:9562:18)
    at Object.parseExprList (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:9536:22)
    at Object.parseExprAtom (/home/circleci/project/node_modules/@babel/core/node_modules/@babel/parser/lib/index.js:8841:32)

Dangerfile

--------------------^

Generated by 🚫 dangerJS against 847034f

[sweir27]

Thanks for adding this! The first time we add a new app from scratch into this framework, we should link to that PR as an example.

Would also love for some more explanation on why we do things a certain way, but I imagine that'll come in your lunch and learn and we can link/add here at that point.

[sweir27]

Not sure what the danger error is-- feel free to merge unless you're looking into that!