Update redirecting links to resolved URLs

What does this MR do and why?

Updates external documentation links that currently return an HTTP redirect so they point directly at their resolved destination URLs.

The changes were generated by the new fix-redirects task in docs-gitlab-com, which follows redirects with lychee and rewrites the source links to their final targets.

⚠️ Reviewer attention

Auto-following redirects produced a few destinations that may be worse than the original link. Please verify (and drop the change if so):

• doc/settings/smtp.md: cp.easydns.com/manage/domains/mail/outbound/ now points at cp.easydns.com/login.php. The original deep link likely redirects to login only because the checker is unauthenticated.
• doc/settings/ssl/_index.md: the two rfc-editor.org/rfc/rfc7540 links now point at the /info/rfc7540/ landing page; the #appendix-A anchor may not exist there.

The remaining changes (trailing slashes, moved blog/package/cloud-init/pulp pages) are straightforward.

Related

• Tooling that generated these changes: gitlab-org/technical-writing/docs-gitlab-com!2338
• Related to Proposal: Autogenerate MRs to clean up out-of-d... (gitlab-org/technical-writing/docs-gitlab-com#635)