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 1666fba8f577e11ea234c8a671aeaab1290cfbaf)
(cherry picked from commit 48f38280e8b9f34d7c45399f05a670ef3460dac1)
(cherry picked from commit 03adb3a2b46081e183738a86ca3d54bf730de0bd)
(cherry picked from commit a0ad36f0ad5d99896d5319e9aca11d0cf0ce23ee)
(cherry picked from commit 3aac9900640da2eeaac7950d14132361923d1a69)