Replace old URLs with new doc URLs in code, README, CHANGELOG.
Critical old URLs, for example those printed by gitlint itself in the past,
will keep working and redirect to the new URL.
Ensure all links are https (no http)
Fix incorrect top-level doc URL path for user defined rules
(rename getting_started.md to index.md).
Replace few accidental uses of jorisroovers.com/gitlint with
jorisroovers.github.io/gitlint
Coverage: 99.661%. Remained the same when pulling 4a813a3551e17ee22a81237110fd6c018e5d8314 on fix-old-links into a19e7d88c3edb145c56af983bdc64be38b36511f on main.