getLedgerEntries: Outline building and parsing all sorts of LedgerKeys

[Shaptic]

This is marked as "ready" to see the preview! Please don't review it fully yet.

This makes the following changes:

1. Add a guide on building + parsing account and trustline entries
2. Improve the guide on building + parsing contract data, instances, and WASMs

As part of (2) I tried to morph it to be in line with the way (1) is explained. Notice that I've removed all of the base64 cruft: this is intentional as these can quickly become out of date and the guide is structured to teach someone to fish rather than giving them a base64-encoded fish.

Things still left to do:

• more references to the Lab and leveraging xdrFormat: "json"
• references to the stellar-cli to encode/decode structures
• curl-esque examples

Closes #469.

For Reviewers

I recommend viewing the entire Markdown file in this PR as-is and comparing it to the existing page rather than going line-by-line as the changes are significant and hard to parse out of a diff.