docs: correct notifications, autocomplete, share visibility and the slim vector default on the getting-started pages

[silentoplayz]

Summary

I compared the "Open WebUI as an app", essentials and sharing pages with the frontend and the config defaults.

• Installed app page: there is no push service or service worker. What exists is desktop notifications for finished responses, turned on per user in Settings > Notifications, and only while the installed page is open. EXTERNAL_PWA_MANIFEST_URL is fetched by the server, and the manifest name already follows WEBUI_NAME.
• Essentials: autocomplete is off by default (ENABLE_AUTOCOMPLETE_GENERATION), so it is no longer listed among the tasks that cost by default, and the :slim image defaults to PGVector rather than Chroma.
• Sharing: a share link is private to the users and groups granted. Instance-wide and no-sign-in links need permissions that are off by default. The community upload button and its ENABLE_COMMUNITY_SHARING switch are named, and so are DEFAULT_USER_ROLE and ENABLE_SIGNUP for the pending queue.

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: src/routes/+layout.svelte (the Notification call), main.py (/manifest.json), config.py (ENABLE_AUTOCOMPLETE_GENERATION, VECTOR_DB, USER_PERMISSIONS_CHAT_ALLOW_PUBLIC_SHARING, USER_PERMISSIONS_CHAT_ALLOW_OPEN_SHARING, ENABLE_COMMUNITY_SHARING, DEFAULT_USER_ROLE), and routers/chats.py for shared chat access.