Add Quick Start section to README

[VNel]

This pull request adds a short "Quick Start" section to the README.

While the README already points users to man htop and the built-in help for a complete list of commands, it currently does not provide a short, immediately visible overview of commonly used actions.

The new section is intended to improve onboarding for new users by highlighting a few frequent default actions (e.g. searching, sorting, tree view, killing a process), while explicitly referring to man htop as the primary and complete documentation source.

No functional changes are introduced.

Definition of Done (DoD):

• A new "Quick Start" section exists in the README
• The section lists only common default actions and key bindings
• Existing documentation remains unchanged
• No functional or behavioral changes to htop
• man htop remains the primary reference for full documentation

Related issue: #1840