Gem warning, page layouts, old pages, general fixes

[Langhaarzombie]

This PR resolves the warning that occurs when building the webpage using GitHub Actions.
The change includes a transition from using the branch based deployment method towards using the Jekyll plugin for GitHub Actions directly.
This closely followed the best practice suggested on the Jekyll website.

As part of this change, the settings for deployment need to be changed. This needs to be done by an admin and is described here.
Essentially, by going to Settings > Code and automation > Pages, the Build and Deployment source needs to be changed to GitHub Actions. This needs to be done after merging, so that the jekyll workflow can be selected.

Let me know if we should do a meeting for this transition!

Also fixes the 404 page and default layout...

[hodgestar]

Thanks! It's nice to have a bit more visibility / control over the deployment process.

[hodgestar]

I've applied the admin settings update.

[Langhaarzombie]

I am turning this into a more general PR cause there are so many small things that are changed.

So to summarize the changes that are performed:

• Fixes the Gem warning on page build (already reviewed)
• Fixes the default layout of pages, 404 page looks good (already reviewed)
• Adds message to dowloads page that it is archived
• Readds the documentation page to the navbar for older versions
• Fixes a href for the supporting organisations which is sometimes used on external pages