Improve ZenBPM documentation

[thomasek]

Description

1. Empty pages still in production navigation. /reference/dmn-engine, /reference/script-engine, and /reference/zenctl contain literally only the title. Please provide meaningful content
2. TODO/Future-plans markers in user-facing reference docs. /reference/bpmn/bpmn-engine has "TODO: add information about how we are parsing xml definitions of processes", an empty "Workers" section, and the Cluster page has "TODO: add grpc endpoint as well" and two "Future plans" admonitions inline.
   If these parts area already implemented, please add meaningful documentation. If not, please remove them and add new issues into GitHub so that we do not forget about them.
   3)Tutorial Step 6 carries a :::warning this feature is not implemented yet.
   This feature is implemented (Process instance history), so please add a meaningful documentation
3. The TODO links at the bottom of the tutorial and the deploy how-to point to /future-feature with a TODO badge. Same problem — looks abandoned. Replace with real links or remove.
4. DMN. Page is empty. You need to add e.g.: DMN element coverage, decision tables, hit policies (UNIQUE, FIRST, ANY, COLLECT, RULE ORDER, OUTPUT ORDER, PRIORITY), DRDs, FEEL inside DMN, deploying decision definitions, evaluating decisions standalone via API, calling decisions from a Business Rule Task.

See the Claude analysis https://claude.ai/share/c75d2f90-03dc-4e67-b2bf-641cfd4171df for more details

Acceptance Criteria

documentation is improved as described above