docs: correct the Python tabs' data directory, and document the serve options, secret key file, and single-worker migrations

[silentoplayz]

Summary

I compared the Python install tabs with the CLI, the env code and the build hook.

• The data directory: all four uninstall sections said the data lives in ~/.open-webui. When started with open-webui serve, it defaults to open_webui/data inside the package, so removing the environment removes it. ~/.open-webui only holds data if DATA_DIR was set to it.
• The Pip tab now names --host and --port. It says that open-webui serve writes .webui_secret_key in the directory it is started from, so starting elsewhere signs everyone out. It also says that installing from source or a Git URL runs the build hook, which needs Node.js and npm.
• The multi-worker update warning now mentions ENABLE_DB_MIGRATIONS=false as the way to keep extra workers from running migrations.

Related issue or discussion

None.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Sources on dev: env.py (DATA_DIR with FROM_INIT_PY), open_webui/__init__.py (serve, KEY_FILE), hatch_build.py, and config.py (run_migrations gated on ENABLE_DB_MIGRATIONS).