docs : Add vLLM Usage : starting-with-vllm

[hhhhhhhhhhhhhhhhho]

Pull Request Description

Target branch: main ✅

---

Description

• Added instructions for **starting with vllm **
• Provides guidance on installing and configuring vLLM, enabling users to easily integrate LLM/RAG functionality.
• No changes to existing features or UI; this is purely a documentation update.

Added

• Add vLLM Usage in quick-start:
  • vllm server setting
  • connection in openwebui

Deprecated

• None

Removed

• None

Fixed

• None

Security

• None

Breaking Changes

• None

---

Additional Information

• This is a documentation-only update; no impact on existing functionality.
• No related issues or prior discussions.

[pr-validator-bot]

ℹ️ Documentation PR Guidelines

👋 Welcome! This is an automated message posted on all new documentation PRs to help guide our contributors. Just because this comment appeared doesn't mean you have done anything wrong!

Please ensure you're using the correct branches:

Target branch (where you're merging TO):

• dev branch: For documentation related to upcoming Open WebUI releases (new features, new environment variables, anything dependent on unreleased versions and unreleased features/fixes/changes)
• main branch: For content that can go live immediately (fixes, tutorials, documentation not dependent on unreleased features)

Source branch (where you're merging FROM):

• If targeting dev, create your branch from your fork's dev branch
• If targeting main, create your branch from your fork's main branch
• ⚠️ Mismatched branches can result in unwanted file changes being included in your PR!

If your docs PR depends on a pending PR in open-webui/open-webui:

• Convert this PR to DRAFT mode!
• Link to the related main repo PR in your description for clarity
• We'll review both together once the PR on the main repo is merged

Please adjust your PR target branch, source branch, and/or draft status accordingly if needed.