Addon-docs: Refactor docs support into individual framework packages

[shilman]

Issue: #16210

What I did

• Created a new library @storybook/docs-tools to contain common functions
• Moved framework into angular, ember, html, react, svelte, vue, vue, web-components
• Cleaned up addon-docs complex preset system

In the telescoping PR #17697

• Added the ability to disable docs support for these packages
• Updated the documentation

What to review

1. Naming: does docs-tools make sense?
2. Overall structure: does the framework update with the preset-framework-docs, which in turn calls into docs-tools make sense? Does this bloat the framework packages in some way that I'm missing, other than what's handled in Addon-docs: Auto-disable docs presets if docs/controls unused #17697?

How to test

I tested this using Storybook test-runner, doing visual snapshot tests against the docs pages in both next and this branch.

You can see the feature/instructions here: storybookjs/test-runner#79

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[tmeasday]

LGTM