feat: install man pages

[puneetdixit200]

Summary

• Install optional man/ directory contents into <prefix>/share/man during source installs
• Add install test coverage for man/man1/<tool>.1 files
• Document local/global man page install destinations and MANPATH guidance
• Use a portable install invocation for bin scripts so the install tests run on macOS and Linux

Closes #7

Verification

• /opt/homebrew/bin/bash -n lib/internal/bashlib_install.bash test/test_lib_install.bash
• git diff --check
• /opt/homebrew/bin/bash ./test/test_lib_install.bash

Known existing issue

• /opt/homebrew/bin/bash ./test/test_all.bash still fails when it reaches create-project tests because lib/internal/bashlib_create.bash is missing on main.

AI disclosure

This contribution was prepared with AI assistance and manually reviewed before submission.