docs: fix broken internal markdown links

[nthbotast]

Summary

• fix stale runs.md references in docs/pipelines.md by pointing to customruns.md
• fix malformed #./workspaces.md link in docs/podtemplates.md
• replace two compatibility policy relative links with stable GitHub blob links

Validation

• git diff --check
• targeted check to ensure old broken link targets are removed and expected replacements are present

Closes #9498

[tekton-robot]

@nthbotast: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[tekton-robot]

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