Fix most instances of bare URLs in markdown

What does this MR do?

Changes most instances of bare URLs (that aren't wrapped) to use < > for true URLs, and ` ` for example URLs, to help the parser parse URLs and allow the markdown to pass this rule in markdownlint: https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#md034---bare-url-used.

Other minor fixes present too. Some changed URLs where I detected existing URLs were being redirected.

Related issues

Relates to: https://gitlab.com/gitlab-com/gitlab-docs/issues/287.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9233.

Review checklist

• Technical writer's review (required)
• Merge the EE-MR (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9233) first, CE-MR afterwards