Split secure coding guidelines by language

What does this MR do and why?

AppSec aims to add dedicated secure coding guidelines for every major project (such as the Duo-related projects). See https://gitlab.com/gitlab-com/gl-security/product-security/appsec/appsec-team/-/issues/1357+ for more info. This MR attempts to split the existing secure coding guidelines per language, so that we can have rules for different languages like JavaScript and Python in a separate page, instead of filling the same page with guidelines for all languages.