Add documentation .md files to test the pages job

I have created a new branch called d10-docpages and pushed an initial commit. This branch does not have any .module or .info.yml or composer.json files. However it does have a mkdocs.yml and docs/index.md.

We can work on the changes neede to test the 'pages' job in due course. But the immediate reason for adding this branch is to allow testing of ✨ PHPCS is run even when it cannot check any files Active

Here's the new repo
https://git.drupalcode.org/project/gitlab_templates_downstream/-/tree/d1...

Although I have just noticed that we had suggested using the 'main' branch for documentation. That may be a better idea, rather than have a new branch. I should have read the issue summary before working on it! I came from the other issue and just did it.

I've opened MR18 to use the 'main' branch. If this works, then when committed we can see if we can delete the new 'd10-docpages' branch. But leave it there until we know that the 'main' branch works OK for this purpose.