docs(web-search): add loader engine guidance

[kayomarz97]

Summary

• add a dedicated Web Loader Engines page for safe_web, playwright, and firecrawl
• link the new page from web search troubleshooting and save-to-knowledge docs
• clarify where loader settings live in the admin panel

Closes #447.

Verification

• npm ci
• npm run build

Notes

• the docs build reports existing broken-anchor warnings elsewhere in the repo, unrelated to this change

[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 and will 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.