[libcxx] Update testing documentation about CI container images.#149192

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cmtice merged 9 commits intollvm:mainfrom

cmtice:libc++-documentation

Aug 6, 2025

Commits on Jul 28, 2025

Merge remote-tracking branch 'origin/main' into libc++-documentation
cmtice
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Move documentation from TestingLibcxx.rst to Contributing.rst
cmtice
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Commits on Jul 29, 2025

Update text to address reviewer comments.
cmtice
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Commits on Jul 31, 2025

Update text as suggested by reviewer.
cmtice
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Commits on Aug 6, 2025

Update spaces as requested by reviewer.
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