docs: fix the contributing tutorial's Pages source, workflow name, tagline, tutorials directory and env variable order

[silentoplayz]

Summary

The contributing tutorial pointed at docs/tutorial/, but the directory is docs/tutorials/. Its step 4 told readers to add BASE_URL and SITE_URL as repository variables before the section that makes the workflow and config read them. The upstream gh-pages.yml build step sets no env, and docusaurus.config.ts hard-codes url and baseUrl. A note now says the variables take effect only after those edits. The example fallback URL is corrected to https://docs.openwebui.com. Step 3 now selects GitHub Actions as the Pages source, since the workflow deploys through actions/deploy-pages rather than from a branch, step 5 names the workflow by its real name, Deploy site to Pages, and the example tagline matches docusaurus.config.ts.

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 in this repository: .github/workflows/gh-pages.yml and docusaurus.config.ts on main.