Issue #3607947: Update the Architecture and Execution mode docs to match the codebase

Two documentation-accuracy fixes for pages that had drifted from the codebase.

Architecture page (four stale inventories; prose sections still check out):

• Kernel entities: now two config entities (Workflow, Tenant) plus the runtime content entities ProcessInstance, Token, Variable, WorkflowVersion and Incident. The page listed only Workflow and a "runtime trio".
• Plugin types: added NodeFeature, TimeoutAction and Audience alongside TaskType, FlowCondition, Split, Join and Gateway.
• Engine methods: added resumeWithResult() and the incident operations (retryIncident, resumeIncident, skipIncident, cancelIncident, failFromIncident).
• Submodule table: added the six missing submodules (orchestra_notification, orchestra_inbox_notification, orchestra_interaction_notification, orchestra_easy_email, orchestra_task_interaction, orchestra_webform_example) and corrected the orchestra_mail row, which described the old task-assignment model rather than the current channel model.

Execution mode page: clarified per-instance mode switching. The mode is stored on the instance (a synchronous flag) and can in principle be changed per instance mid-run, but there is no built-in control (no UI, bulk action or drush command); today it is reachable only from code via ProcessInstance::setSynchronous(). The page previously implied it was switchable without saying how.

Verified with mkdocs build --strict.