Update http:// to https:// in links

Documentation location/URL

There are many instances of links using http://, which could be updated to https://, since that's the standard now.

Problem/Motivation

Using http:// makes the manual look outdated.

Proposed resolution

Search-and-replace http:// with https:// everywhere:

https://git.drupalcode.org/search?search=http%3A%2F%2F&project_id=55782&...

It should be safe to assume that this is all right in all instances.

Remaining tasks