* index plug: index paragraph nodes
Includes attributes and tags in the paragraphs.
Only considers top level paragraphs (right below Document node)
* keep ref and pos consistent with other objects
- most objects just store the starting position in the `pos` field.
- ref is usually `${page}@${pos}` so that it works as a link.
* cleanup and clarify comments
* add paragraph to builtins index
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