[heycam/webidl] Mixin follow-up (#450)

- [ ] Reorganize content:
    - [ ] Move namespace up after Mixins and before Members.
    - [ ] Split Interface section into the following subsections:
        - interface
        - inheritance
        - callbacks
        - partial interface
    - [ ] Give subtitles to grammar sections?

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https://github.com/heycam/webidl/issues/450