[w3c/webcomponents] Fix HTML Modules demo links (#803)

Add HTML Modules demo.  Fix links in HTML Modules explainer doc referencing the demo.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/803

-- Commit Summary --

  * Fix HTML Modules demo links

-- File Changes --

    A proposals/HTML-Modules-Demo/HTML5-Element.html (43)
    A proposals/HTML-Modules-Demo/Main.html (13)
    M proposals/html-modules-explainer.md (4)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/803.patch
https://github.com/w3c/webcomponents/pull/803.diff

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