docs(tutorial): Update plugin tutorial steps for NetBox v4.5

[pheus]

Fixes: #36

Thanks for taking a look.

This PR updates the NetBox plugin tutorial for NetBox v4.5 and makes the steps a bit easier to follow, especially for readers who are new to Django and NetBox plugin development.

Key changes:

• Split the initial setup into its own dedicated step
• Refresh all tutorial steps for NetBox v4.5, including:
  • decorator based view registration and get_model_urls() routing
  • tables, forms, templates, navigation
  • filter sets and filter forms
  • REST API and GraphQL
  • global search indexing
• Improve wording and add practical tips to reduce common setup mistakes

Also addresses:

• Include an example using a top-level navigation menu (v3.4) #22
• Demonstrate adding a custom tabbed view to a core model (v3.4) #23
• Rewrite the first step of the tutorial to use cookiecutter-netbox-plugin #24

[pheus]

Thanks for reviewing this!

Once this PR is approved, could you please hold off on merging it for now? I still need to update the demo repository to match the final, approved tutorial content. I will follow up here as soon as the demo repo update is ready and it is safe to merge.