Building off [this Slack conversation](https://vercel.slack.com/archives/CGD3XGSD7/p1597329727013900), this PR adds a top-level section to the documentation on authentication patterns.

Please provide any and all comments! A few open thoughts I have:

- ~Should this include code snippets from the related providers or stay very high-level? At what point do we delegate to the examples folder?~ Keep things high level and delegate to examples folder 
- ~Should this include any related cards at the bottom?~ Added to the bottom
- ~Should other places in the documentation link back to here?~ Added link from routing
- Should it be a top-level route, or be underneath advanced?