Create a plugin migration guide for NetBox v4.0

[jeremystretch]

Change Type

Addition

Area

Plugins

Proposed Changes

Add a migration guide to the plugin development documentation detailing relevant changes in NetBox v4.0 for plugin maintainers. Example topics include:

• Any breaking changes to the plugins API
• Relocation/removal of internal components
• HTML template changes
• UI design changes

[peteeckel]

Some input from the migration of NetBox DNS to 4.0:

peteeckel/netbox-plugin-dns#153

This PR will be updated over time, following the development of the NetBox feature branch.