[webcomponents] Group registry related terminology under a heading (#437)

Currently it is under the "Custom Elements" heading (should maybe be "Custom elements" to match style of documents it'll be merged into). However, the paragraphs that describe it are not all adjacent. I guess this is done to first introduce the concepts before the other concepts that reference them, but it makes it harder to figure out what the model of a registry is.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/437