[WIP] Update IA and summary

[lbv-stripe]

This PR:

• Fixes a few confusion points in the IA (rendering and validation aren't part of syntax, common examples aren't an integration guide, config objects deserve their own section).
• Makes sure that the Node, Tag, etc articles are complete. Moves information that only existed in Syntax into those articles.
• Adds information about annotations to the Attributes page, since that's basically what annotations do.
• Moves built-in features to the tops of their pages — e.g. built-in tags are now higher than info about custom tags. (This makes it less painful to move some of that info out of Syntax.)
• Does a bit of rewriting and adds a few examples to support all this.

This solves most of the problems in https://jira.corp.stripe.com/browse/COSTRAT-76, plus some extra cleanup.

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[mfix-stripe]

Chatted w/ @lbv-stripe offline. We are going to split this up into smaller changes 🙂

[mfix-stripe]

I strongly think we should keep /docs/render in the first section. With that, I think "Core concepts" is a better fit, but I could be convinced about "Core features".

[mfix-stripe]

Either way, for changing concepts like this, I think we need some outside data (even if it's a collection of anecdata), before changing IA stuff.

A lot of this comes down to personal opinion and personal learning styles, so we want to be more objective.

[lbv-stripe]

Going to break this into smaller PRs