Add Tutorials site reference and learning path

[michaellwest]

Scope

Update training pages to link to the interactive tutorials site (sitecorepowershell.com tutorials). Briefly describe what it offers:

• 29 interactive browser-based lessons
• Simulated Sitecore tree (no instance needed)
• REPL and ISE modes
• Optional live Sitecore connection

Add to community.md as well.

References

• SitecorePowerShell/tutorials (repo)

Files

• training/next-steps.md
• training/README.md
• community.md