Partially revert "Refactor the sphinx-dev build environment to get rid of the link-stamp step"

This reverts commit 457c9f64.

This commit removed both the source copy step and the actual linking of the docs sources. Turns out that the latter is "somewhat" necessary for a proper doc build.

Close #2386 (closed)

Test Plan

tox -e sphinx /and look at the result/

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Migrated from D3097 (view on Phabricator)