docs(user): add Flows add-on user guide (en/de)

[MartinHartkopf]

What

Adds a Flows section under the User guide (English and German) documenting the Flows automation add-on for i-doit up. Flows appears under the Add-ons menu in the app, so this fills the gap in the user-facing docs.

Each flow is a linear chain Trigger → Conditions (optional) → Action(s). The section is structured as:

• Overview (user/flows/index.md) — concept, interface, creating a flow, access & rights
• Triggers, conditions, and actions (reference.md) — reference tables of every building block + placeholders
• Trigger / Condition / Action use cases — one example per type, each with a screenshot
• End-to-end example — a complete button → Call API webhook flow with a logged, successful run

Details

• New pages under docs/en/user/flows/ and docs/de/user/flows/ (6 pages each).
• Screenshots under docs/{en,de}/img/screenshots/flows/, taken from a live i-doit up 3.1.0 instance.
• Nav entries added under User guide → Flows in i18n/en/mkdocs.yml and i18n/de/mkdocs.yml.
• Dictionary words added to i18n/{en,de}/spelling.txt for the spell checker.
• Pages containing {{ … }} placeholders set render_macros: false so the macros plugin leaves them literal.

Validation (run locally with the repo toolchain, Node 24 + venv)

• cspell (en + de): 0 issues
• alex: no issues (de is excluded via .alexignore, as per repo)
• mkdocs build strict for en and de: builds cleanly
• yamllint on the nav files: clean
• remark: only the same style warnings the existing docs emit (non-fatal)

🤖 Generated with Claude Code