Fix live sample IDs for CSS docs

[wbamberg]

This is part of #5865.

On MDN, live samples (the EmbedLiveSample macro) are passed an ID that Yari uses to find the code to include in the sample. These IDs typically (almost always, in Markdown-land) refer to headings.

In HTML, we can have explicit id attributes for headings, that can differ from the ID that would be auto-generated from the heading's text. Live samples can then use these "custom" IDs to refer to the scope where Yari can find the code.

In Markdown we can't do that - IDs are always generated from the heading text. So if a live sample in the old HTML system was using a custom ID, this will cause an error when we convert to Markdown.

I've checked through the CSS docs to find pages where this was happening - effectively where a live sample was using an ID that differed from the one that would be generated by Yari's slugify function. In this PR I've fixed all the macro calls to use the auto-generated form.

To make it easier to check that this is working, I've deleted the explicit id attribute in all affected files, so all EmbedLiveSample calls will need to use an ID that matches the generated one. Note that in some cases here the macro was already using that form, so I just deleted the id in the heading. In other cases I've correspondingly adjusted the macro argument.