Remove sidebar pane if there is no sidebar

[selfthinker]

When using the core layout, the standard sidebar isn't used and you can put anything in it. But if you don't use the sidebar (with content_for :sidebar) at all, the empty pane still shows and the content is still further to the right.

This fixes that by removing the sidebar pane if there is no sidebar.

There is a slight chance that this is not 100% backwards compatible in case someone wanted no sidebar content but still wanted the pane to show. I assume that's extremely unlikely, though.

[selfthinker]

One downside is that the issue of alphagov/tech-docs-template#123 becomes more apparent with this change as you cannot click the "Menu" toggle in mobile view anymore when the "Table of contents" is missing to add the required space.

[selfthinker]

I have fixed that unclickable Menu toggle (although it only partly fixes the other linked issue).

[selfthinker]

Adding an example page makes sense, I can do that.

I initially had an alternative way of doing this via a frontmatter variable. I added something to the documentation in that case. But when I did this version instead, I removed the documentation again, because this is more of a bug fix than a new feature. At least I assume that most people would expect a sidebar to not be there if they didn't add any sidebar. That's why I think documenting this behaviour would be confusing.

[selfthinker]

Actually, because the example in the documentation only has one example for when you put something else into the sidebar, adding something with a non-existing sidebar does make sense. I'll do that.

[selfthinker]

Done. I've added the example page, updated the documentation and added a test (not just for the lack of a sidebar but also to test if there is a sidebar on the homepage).