Update contribtion docs for new mixin guideline

[Elchi3]

See #1940 for background.

[sideshowbarker]

Not merging since as project-meta docs, it seems this should have review and OK from @chrisdavidmills

[Elchi3]

Thanks Mike and yup, Chris said yesterday he'll have a look.

[chrisdavidmills]

Mostly looking good here, Mr @Elchi3 ! I just had one bit of feedback for you.

Changelog:

• The content you've added here is not wrong, but it feels a bit thin on the ground. It feels to me like you should provide a bit more information — what it used to be like and what it is like now — and link to the document section that explains how to document a mixin.

So something like "Previously we commonly defined a landing page for the mixin class itself, and put the defined members on subpages underneath it, before linking to those from the landing pages of the interfaces that implement those mixins. This was confusing for readers because mixins are spec constructs — you never access the defined members using the mixin classes. To avoid this confusion we've instead put the pages for members defined on mixins directly under the implementing class pages. For more details, see XXXXXXX"

[chrisdavidmills]

Much better, thanks @Elchi3 !

So, small copy edit, and we're done. Merging now.