It will determine how anchors are created and will break existing
links otherwise.
Adapted from Revert "Make `user-content-* ` consistent with github (#26388)
(cherry picked from commit 1666fba8f5)
(cherry picked from commit 48f38280e8)
(cherry picked from commit 03adb3a2b4)
(cherry picked from commit a0ad36f0ad)
(cherry picked from commit 3aac990064)
(cherry picked from commit 137daabc9b)
b438aed4c1