docs: document default OpenAPI schema endpoints

[Brohammad]

Summary

• Adds a Default schemas section to the OpenAPI usage docs explaining where to download the generated schema
• Documents that GET /schema/openapi.json is always available by default (via automatic JsonRenderPlugin registration)
• Clarifies that YAML endpoints require explicit YamlRenderPlugin configuration and the litestar[yaml] extra
• Cross-links to the UI plugins docs for custom schema paths and YAML setup
• Notes route conflict behavior, 404 handling for unknown schema paths, and programmatic access via app.openapi_schema

Fixes #3941

Context

This addresses the same issue as #4830 with corrections for factual accuracy (YAML is not served by default, which is covered by test_openapi_yaml_not_allowed in the test suite). Happy to close this in favor of updating #4830 if maintainers prefer.

Test plan

• pre-commit run --files docs/usage/openapi/index.rst
• pre-commit run --all-files
• sphinx-build -b html -n docs docs/_build/html (no warnings on changed page)
• CI green on GitHub

---

📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4908

[Copilot]

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[Brohammad]

@all-contributors please add @Brohammad for doc

[allcontributors]

@Brohammad

I've put up a pull request to add @Brohammad! 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.29%. Comparing base (bd18fd4) to head (4a05808).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4908   +/-   ##
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  Coverage   67.29%   67.29%           
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  Files         293      293           
  Lines       15228    15228           
  Branches     1728     1728           
=======================================
  Hits        10248    10248           
  Misses       4833     4833           
  Partials      147      147           

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[Brohammad]

@all-contributors please add @Brohammad for doc

[allcontributors]

@Brohammad

@Brohammad already contributed before to doc

[Brohammad]

All checks pass, no conflicts, and review feedback is applied. Ready to merge when convenient.

[Brohammad]

@codex please review this PR.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: ca081072e5

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[Copilot]

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[provinzkraut]

@Brohammad Your behaviour of opening, closing, re-opening PRs, not taking into account feedback, and submitting redundant PR is highly disruptive. Furthermore, you repeatedly ignore our AI policy. You'll be blocked from further contributions in this organisation