Push annotation and attribute info into attributes.md

[lbv-stripe]

Right now, we've got two related problems:

• The IA implies that /docs/attributes is the most detailed source on attributes — but some attribute info is only in /docs/syntax, making the whole story hard to find
• The structure of /docs/syntax implies that annotations are a separate feature — but there's no /docs/annotations, making the whole story hard to find

This PR addresses those by pushing most attribute and annotation info into /docs/attributes, and by adding language that clarifies that annotations are just an alternate syntax for passing attributes.

[vercel]

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Name	Status	Preview	Updated
markdoc	✅ Ready (Inspect)	Visit Preview	Oct 31, 2022 at 6:27PM (UTC)
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