HTTPSify links (PR #7046)

Upgrade all links to HTTPS in Discourse’s documentation.

Note to Discourse team: Diff for the whole PR lives at https://github.com/discourse/discourse/pull/7046.patch or at https://github.com/discourse/discourse/pull/7046/files.

You might want to squash merge this with a custom commit message and commit body since there are a lot of commits.

GitHub

You’ve signed the CLA, sau226. Thank you! This pull request is ready for review.

@gschlager Is there any action that needs to be taken on my side or are we good to go?

Thanks!