docs: Create user-facing documentation for hooks/plugins feature

[ajbozarth]

Problem

The hooks/plugins feature is fully implemented but has no user-facing documentation. Only a design document exists in docs/dev/hook_system.md.

Why not a simple move?

docs/dev/hook_system.md is written as a design document and needs adaptation:

• Remove "Design Document" framing and implementation details (CPEX references)
• Convert from architecture tone to how-to/tutorial tone
• Add practical examples and use cases
• Follow published docs conventions (frontmatter, no H1 in body, markdownlint compliance)

Files involved

• Adapt: docs/dev/hook_system.md - Contains good explanations but needs user-facing rewrite
• Remove: docs/dev/hook_system_implementation_plan.md - Internal implementation details

Tasks

1. Create new published docs page(s) for hooks/plugins feature
2. Adapt content from docs/dev/hook_system.md (remove CPEX details, add examples)
3. Ensure content follows published docs conventions
4. Remove both dev docs after migration

[planetf1]

@araujof added docs in #582 on hooks and plugins and we discussed linking into the index.
Working