Convert all shell snippets to explicit code blocks

[battaglr]

By explicitly declare shell snippets as code blocks, we:

• Ensure syntax highlighting.
• Avoid that future updates on the Markdown parser ignore spaces as code markers.

Other minor changes have been made:

• Add missing $ characters.
• Remove unnecessary whitespace.
• Add missing whitespace.
• Update year in the copyright notice.
• Limit line length to 80 characters or less (when possible).