docs: add reproducible example for generating man page

[benslimanh]

Fixes #20699

This PR addresses the documentation gap in manpage.md by providing a clear, reproducible step-by-step example for generating the curl.1 man page.

Changes included:

• Added the exact manual invocation commands using the managen script for both nroff and plain-text outputs.
• Clarified the usage of the -d and -I flags.
• Added a clear explanation of how the script expands the %options keyword, as well as the %VERSION, %DATE, and %GLOBALS variables.

This should significantly lower the barrier for new contributors trying to build the man pages locally.

[bagder]

@sammydono I take it this satisfies your curiosity?

[bagder]

Thanks!