Addon-docs: Add docsContainer parameter

[shilman]

Problem

SB Docs has a DocsContainer component that adds a bunch of context providers including: DocsContext, Theme, MDX, as well as some global page formatting. Most Doc Blocks need this context to function.

We can't add this wrapper in @storybook/core because it adds a bad dependency. Therefore it's automatically inserted by the MDX compiler and also included inside the DocsPage component.

However, the current structure is limited in the following ways:

1. Duplicate code between compiler & DocsPage
2. No easy way to use Doc Blocks in arbitrary docs override e.g. CSF Stories with MDX Docs recipe not working #7810 -- user would also need to add this code
3. Inflexible: there's no easy way to override the wrapper if needed

Solution

• Refactor DocsContainer into a docsContainer parameter (dependency injection)
• Have presets set this parameter
• Update manual config docs
• Update tests
• Update DocsPage & MDX compiler
• Clean up API in the process (content prop => children)

[shilman]

Egads!! I just released https://github.com/storybookjs/storybook/releases/tag/v5.2.0-beta.37 containing PR #7814 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.