Docs: document custom workspace builders

[tony]

Summary

Add developer documentation for workspace builder classes and custom builder integration.

Related: #1049, #1054, #1058.

Proposed content

• Explain how tmuxp loads a workspace config and hands the expanded workspace to a builder.
• Document the classic builder default and how to select a custom builder by dotted path.
• Document optional trusted import paths for user config directories, including why tmuxp avoids site-directory startup processing for this feature.
• Document package entry points once the builder protocol/entry-point stage exists.
• Show small custom builder examples that use libtmux object APIs, raw tmux commands, and normal Python code.
• Explain required builder behavior around plugin lifecycle hooks, progress callbacks, script output, append/detached behavior, and session exposure.
• Include testing guidance and examples for custom builders.
• Explain when users should choose classic behavior, readiness tuning, or an experimental/parallel builder.

Acceptance criteria

• Docs include a working minimal custom builder example.
• Docs include a configuration example using workspace_builder and trusted import paths.
• Docs explain the security/trust boundary for importing custom Python code.
• Docs cross-reference the builder protocol and pane readiness configuration once those stages land.

[tony]

Shipped in v1.73.0 (#1066). Added a custom workspace builders guide (docs/topics/custom-workspace-builders.md) covering builder selection, the trust boundary for importing builder code, pane-readiness tuning, and how to test a builder, plus an API reference grid for the tmuxp.workspace.builder package.