docs: explain updating and deleting models on the Ollama page

[silentoplayz]

Summary

The Ollama page covers downloading and unloading models, but not the other two things the Manage dialog does: Update All Models and Delete a model. This adds an "Updating and Deleting Models" section between downloading and unloading.

It covers:

• that both act only on the Ollama server behind the connection whose Manage was opened
• Update All Models: it re-pulls every model one at a time, shows progress with a Cancel button, skips a model that cannot be pulled, and ends with "All models are up to date"
• that a model loaded before an update keeps the old version until it is unloaded, with a link to the unloading section
• Delete a model: the steps, that it frees disk space and removes the model for every user of that connection, and the Model IDs (Filter) as the way to hide a model without deleting it

Related issue or discussion

None. Model updating and deleting was listed as missing in #2 (comment) ("Pull/Update/Delete Models").

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

Checked against dev at 015dbc8: updateModelsHandler, deleteModelHandler and the dialog markup in src/lib/components/admin/Settings/Models/Manage/ManageOllama.svelte, and delete_model in backend/open_webui/routers/ollama.py, which deletes on the connection index the dialog passes.

The page built and rendered with docusaurus start, and the new section's links to Step 2, Advanced Configuration and Unloading Loaded Models resolve.