Revert "Underscore in markdown should be escaped with backslash [ci skip]"

This reverts commit b58f3a641795e1777aa3e12a853c34ff512acfb9.

Reason: Discussion in https://github.com/rails/docrails/commit/b58f3a641795e1777aa3e12a853c34ff512acfb9

[ci skip]
Revert "Fix on-site markdown rendering [ci skip]"

This reverts commit 427db6b9d2b35a72f3c017eb19a2e1e800b0a7a3.

[ci skip]
Revert "Update security.md"

This reverts commit f4804fafecdc057988575b4516afe9ca1d5f42fc.

[ci skip]
Revert "Update contributing_to_ruby_on_rails.md"

This reverts commit 41ba51f4850a8cd2fe69789011ac33366366d32f.

[ci skip]