docs: Document micromamba shell command and fix CLI help text

[Copilot]

The micromamba shell command lacked proper documentation and had misleading CLI help text. This PR addresses the issue by:

CLI Help Text Improvements

• Main command: Changed description from "Generate shell init scripts" to "Launch a shell or manage shell initialization scripts" to accurately reflect the dual functionality
• Hook subcommand: Improved description from "Micromamba hook scripts " to "Output shell hook scripts for environment activation"
• Enable long path support: Fixed incorrect description from "Output deactivation code for the given shell" to "Enable long path support on Windows"

Comprehensive Documentation Added

Added detailed documentation to the user guide covering:

Shell Command Overview

Explains that micromamba shell serves dual purposes:

1. Launching shells in specific environments
2. Managing shell initialization scripts

Usage Examples

# Launch a shell in the specified environment
micromamba shell -p /path/to/env
micromamba shell -n myenv

# Generate activation code for bash
micromamba shell activate -p /path/to/env -s bash

# Use in a script
eval "$(micromamba shell activate -p /path/to/env -s bash)"

Complete Subcommand Documentation

• init - Add initialization scripts to shell RC files
• deinit - Remove initialization scripts from shell RC files
• reinit - Restore initialization scripts in shell RC files
• activate - Generate activation code for specified environment
• deactivate - Generate deactivation code to leave current environment
• reactivate - Generate reactivation code to refresh current environment
• hook - Generate shell hook scripts for environment activation
• enable_long_path_support - Enable long path support on Windows

Supported Shells

Documents all supported shells: bash, zsh, fish, powershell, cmd.exe, xonsh, tcsh, dash, nu, posix

Common Options

Complete documentation of -s/--shell, -p/--prefix, -n/--name, -r/--root-prefix options

The changes are minimal and targeted - only 3 lines changed in source code plus comprehensive documentation. This makes the micromamba shell command much more discoverable and usable, especially in CI/CD environments and Docker containers.

Fixes #2072.

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