docs: describe the rebuilt slim image, its size, and what to configure instead

[silentoplayz]

Summary

The Docker tag page describes slim as the standard image minus the pre-downloaded model files, at about 1.5 GB, with identical packages, ffmpeg and pandoc. The dev builds after v0.11.3 rebuilt slim from the ground up. It now leaves out every local machine learning runtime and the packages that depend on one, and the compressed download is about 170 MB per architecture.

This PR rewrites the slim sections of that page for the rebuilt image. The variant table row says what slim is for. An info admonition says this describes what :dev-slim serves today and what :main-slim becomes with the next release. It also says :main-slim is the previous slim until then, so the page stays true for someone pulling it this week. A table lists what is left out and what to configure instead, and the saving section now says slim always saves the download, since there are no models left to fetch.

Related issue or discussion

None. The source is the rebuild itself. open-webui/open-webui@cb942bb added requirements-slim.txt and the slim build path, and open-webui/open-webui@a1c0209 reworked the image layers, both on 2026-09-06.

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

Sizes are compressed download sizes read from the registry manifests on 2026-09-10:

Tag	amd64	arm64
dev	1578 MB	1421 MB
dev-slim	169 MB	168 MB
main-slim (v0.11.3)	1472 MB	1315 MB

Every row of the "what to use instead" table was checked against the slim-conditional code on dev. That is the embedding, reranking and vector database branches in retrieval/utils.py and retrieval/vector/factory.py, the loader fallback in retrieval/loaders/main.py, the VECTOR_DB default in config.py, the storage imports in storage/provider.py, and the package list in requirements-slim.txt. The two 503 messages quoted in the table are the ones the code raises.