Add documentation for new zebra-utils scripts and binaries

[oxarbitrage]

https://github.com/ZcashFoundation/zebra/blob/main/zebra-utils/README.md

There are a few new utilities that we need to document here:

• https://github.com/ZcashFoundation/zebra/blob/main/zebra-utils/zcash-rpc-block-template-to-proposal - https://github.com/ZcashFoundation/zebra/tree/main/zebra-utils/src/bin/block-template-to-proposal
• https://github.com/ZcashFoundation/zebra/tree/main/zebra-utils/src/bin/search-issue-refs

[teor2345]

If we're having trouble keeping the README up to date, we could link to the documentation in the source instead. It's closer to the code, so it's more likely to be updated.

For example: https://github.com/ZcashFoundation/zebra/blob/main/zebra-utils/src/bin/block-template-to-proposal/main.rs#L1-L5