Create and document a Testnet reset script for Horizon example responses

[ElliotFriend]

When we remove the Mainnet Horizon URL from the API response examples, any href/URLs in them will give 404 errors when someone visits them. It's probably a pretty rare edge-case, though. Someone would have to highlight/copy/paste the URL to actually come upon that.

Regardless, we should setup a script to run after every Testnet reset to make as many of those example response URLs valid. As a bonus, it can be used as an opportunity to document/demonstrate to users a method by which Testnet accounts/assets/etc. can be re-created when needed. We recommend that practice anyway, but we don't really provide any examples (that I know of).

Refs: #502

[briwylde08]

Related: #608