add(book): Describes custom Testnets in the Zebra book

[arya2]

Motivation

We want to document Zebra's custom Testnets in the Zebra book.

Closes #8588.

Solution

• Adds a page to the Zebra book about custom Testnets and how they compare to Mainnet, the default Testnet, or Regtest.

Related changes:

• Adds the expected genesis_hash to Zebra's configuration.
• Uses the default Testnet if a testnet_parameters section is included in the config with default testnet values and no network name.

Tests

Updates the latest stored config with a configured genesis hash.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.