09-threads.md - add basic mutex and waiting demonstration

[taikedz]

Changes

Add a basic example extending the existing one:

• Uses the same sleepy function
• Adds a global integer counter
• Adds use of a mutex example
• Adds use of a wait group and thread pool for waiting for completions
• Uses defer on mutex and waitgroup as good practice demonstration
• Code is commented, minimally
• Some description of the "why" of the various elements, without expanding on general thread-safety details; this exists in more general tutorials web-wide.

Scope

Updated the examples for 0.13.0 and 0.14.0 directly (I could not find a shared template?). Specifically did not choose to attempt to back-port.

Issues

• uses a sleep() to avoid reaching the waitAndWork(&wg) too soon. Feels like a hack, but not sure what else to do.
  • resolved by calling wg.start() in the main thread

Tested with

• 0.13.0
• 0.14.0-dev.2441+3670910f2

[taikedz]

Done updating and fixing typos, will refrain now from making further changes.

Thank you for Zig.guide , it's a fantastic resource 😄