maint: update copy of `zinit.1` in `$ZPFX/man/` if out-of-date

[psprint]

Description

Keep zinit.1 up to date by comparing timestamps of ~ZIBIN/doc/zinit.1 and of $ZPFX/man/man1/zinit.1 and copying if the ~ZIBIN one is more recent.

Motivation and Context

I've updated zinit.1 when changed README.md, and noticed that $ZPFX/man isn't updated. I've used pandoc from gh-r, the latest.

Related Issue(s)

381

How Has This Been Tested?

I've touched doc/zinit.1 and started a new zsh.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[github-actions[bot]]

:tada: This PR is included in version 3.8.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: