[CI] Fix Doc-Preview GCC 11 runtime compatibility (#79617)

* [CI] Fix Doc-Preview GCC 11 runtime compatibility

* [Doc] fix toolchain PPA setup in doc image

* [Doc] configure toolchain PPA without Launchpad API

* [Doc] vendor toolchain PPA signing key

* [Doc] use pinned GCC runtime image

* [Doc] install libstdc++ from toolchain PPA

* [Doc] clarify bmm broadcasting behavior

* Revert "[Doc] clarify bmm broadcasting behavior"

This reverts commit 7955d5628cf61d224c1d212da0433f27d07446fa.