Makes some http links across 'Language' documents https

[MorayJ]

For format L<name|url> in 'Language' docs, changes http to https where appropriate.

The problem

This is a start on the following:
Use HTTPS URLs, where available #2246

Solution provided

I've switched http to https in the 'Language' docs where the format is L<name|url>

https://vimdoc.sourceforge.net/htmldoc/digraph.html is not correctly configured for https so left as http

https://vim.wikia.com/wiki/Entering_special_characters redirects to a non-secure version so have left as http

[zoffixznet]

I sent you an invite to perl6 GitHub org. You can accept it here: https://github.com/perl6/

And once you do, you should be able to merge this PR yourself and commit any further changes directly.