Add MkDocs documentation site

The module currently has a single README.md that covers basic usage but is outdated (still references D8/D9, contains stale TODOs) and doesn't document the architecture, API, hooks, or configuration in depth.

Add a MkDocs-based documentation site under docs/ with the following pages:

Proposed structure

• index.md — Overview, features, requirements (Drupal 10.3+/11, Workflows, Webform)
• installation.md — Composer install, enabling the module, creating a workflow with the "Webform workflow" type
• usage.md — Adding the element to a webform, configuring states/transitions, access controls per transition, conditional logic to disable transitions, color configuration for states
• email-handlers.md — Setting up the "Workflow transition email" handler, available tokens ([webform_workflow:transition-url:*], [webform_workflow:transition-link:*], secure token variants)
• submission-filtering.md — Filtering results by workflow state (built-in list builder and webform_better_results integration)
• api.md — WebformWorkflowsManager service methods, dispatched events (WebformSubmissionWorkflowTransitionEvent), alter hooks (hook_webform_workflow_element_access_alter, hook_webform_workflow_element_transition_access_alter)
• architecture.md — OOP hook classes (EntityTypeHooks, FormHooks, ThemeHooks, TokenHooks, ResultsFilterHooks, WebformWorkflowsElementHooks), service wiring, #[Hook]/#[LegacyHook] pattern for Drupal 10.x/11.x compatibility
• maestro-integration.md — Maestro module integration (if maestro is installed)
• changelog.md — Migration notes for 2.1.0 (OOP migration, deprecated procedural functions)

Setup

• Add mkdocs.yml at the module root
• Use the Material for MkDocs theme
• Migrate relevant content from README.md into the appropriate pages and modernize it (remove D7/D8/D9 references, update stale TODOs)