fix(docs): Fix NU6-related documentation

[arya2]

Motivation

We want to update the documentation around NU6 funding streams now that draft funding allocation ZIP has been assigned a ZIP number.

We also want to avoid the panics listed as examples in the audit report that could arise from incorrect network parameters.

Note that there are many more potential panics that could be triggered with incorrect network parameters, and ideally those (and many other panics in Zebra) could be replaced with errors, but this should be done when cleaning up tech debt. Leaving these panics should be okay for now, as the hard-coded Mainnet and Testnet parameters have been manually verified, any configured Testnet parameters should be validated when parsing the Zebra's configuration, and avoiding panics in these cases would still leave Zebra unable to correctly validate blocks and transactions, so the benefit of replacing them with errors is limited.

Closes #8903.
Part of #8904.

Solution

• Removes panics listed as examples in audit report
• Fixes outstanding documentation issues listed in audit report
• Addresses outstanding documentation TODOs added during NU6 implementation (except a couple in tests that are waiting on a Mainnet NU6 activation height)

Related fixes:

• Addresses an old TODO in subsidy_is_valid() related to using the correct slow start interval on configured Testnets

Follow-up Work

Replace other panics in Zebra where it would be better to return an error.

PR Author's Checklist

• The PR name will make sense to users.
• 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.