docs(quickstart): add generic OpenAI-compatible endpoint section

[PerryLink]

Summary

Adds a short "Use another OpenAI-compatible endpoint" section to
QUICKSTART.md with a worked example against the official DeepSeek API
(config-only, no code changes), and notes in docs/configuration.md that
the [llm] section accepts any OpenAI-compatible endpoint through
base_url.

Area

• Architecture method
• Benchmark
• Use case
• Documentation
• Developer experience
• CI, build, or release

Verification

Docs-only change; no build or test impact. The DeepSeek base URL
(https://api.deepseek.com) and model id (deepseek-flash) were verified
against the official DeepSeek API docs (https://api-docs.deepseek.com/).
The TOML snippet uses the exact `[llm]` field names documented in
docs/configuration.md.

Checklist

• I kept the change scoped to the relevant area.
• I am opening this from a separate branch, not pushing directly to main.
• I updated docs, examples, or setup notes when behavior changed.
• I added or updated tests when the change affects behavior.
• I did not commit secrets, .env files, dependency folders, or generated output.
• Active relative links in Markdown files resolve.

Notes for Reviewers

Pure documentation change; no code paths touched. DeepSeek is used as one
concrete, verifiable instance of the generic OpenAI-compatible pattern;
the section itself is provider-agnostic.

By submitting this pull request, I agree that my contribution is licensed under
the Apache License 2.0.