fix(docs): sample configurations in the custom testnet book page

[oxarbitrage]

Motivation

I tried to launch some custom testnets and i found a few small issues in the docs, with the sample configurations provided.

Solution

Small modifications so the provided configs can be just pasted into a file and used to launch zebra. Initial peers field was added, with an empty list or:

error: zebrad fatal error: parse error: cannot use default initials peers with incompatible testnet for key `network` at line 44 column 1

Tests

Manual, both configurations now are valid and start zebrad.

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.

[arya2]

Thank you!!