Add Plaid config to new "Bank Sync" settings UI

Current link to `.md` file is temporary, as we'll follow up with embedding the plaid configuration here, which would potentially remove the need for (some sections of) this guide, if we can embed the relevant information in the app.

_Originally posted by @amrawadk in https://github.com/we-promise/sure/pull/111#discussion_r2277473737_