docs: fix four dead external links (SGLang, Substack, Langtrace, OpenLIT)

[yzxcj797]

Four external links 404 because the target sites restructured. All replacements verified live (200):

1. learn/programming/language_models.md - SGLang install page moved from sgl-project.github.io/start/install.html to docs.sglang.ai/docs/get-started/install.
2. community/community-resources.md - the Stephen's Substack post moved when the custom domain was dropped: substack.stephen.so/... -> stephenso.substack.com/p/why-im-excited-about-dspy.
3. community/community-resources.md - the Langtrace blog announcement URL is gone (langtrace.ai/blog/...); pointed at the live Langtrace docs page for DSPy support, which is the same topic.
4. community/built-with-dspy.md - OpenLIT integration docs restructured: /latest/integrations/dspy -> /latest/sdk/integrations/dspy.

[greptile-apps]

Greptile Summary

Refreshes external documentation links for SGLang, Langtrace, and OpenLIT, and removes an obsolete community resource. The affected documentation pages build successfully and render the intended updated links.

Confidence Score: 5/5

No blocking failure remains.

The changed documentation pages were built and their rendered external links were verified successfully.

T-Rex Logs

What T-Rex did

• Before-change MkDocs build completed and rendered-page assertions for base links were executed.
• After-change MkDocs build completed and verified the two updated external URLs are present and the old URL is removed in the rendered HTML.
• Build output included site-wide warnings about unlisted nav pages, Griffe annotations, and live-stat fallbacks, with no changed-page failure or link-rendering error.

_{Ran code and verified through T-Rex}

_{Reviews (2): Last reviewed commit: "remove article" | Re-trigger Greptile}

[isaacbmiller]

thanks for the PR!