Addon-docs: Remove DocsPage slots

[shilman]

In SB5.2, we introduced the concept of DocsPage slots for customizing the DocsPage.

In 5.3, we introduced docs.x story parameters like docs.prepareForInline which get filled in by frameworks and can also be overwritten by users, which is a more natural/convenient way to make global customizations.

We also introduced introduced Custom DocsPage, which makes it possible to add/remove/update DocBlocks on the page.

These two mechanisms are superior to slots, so we should remove slots as a 6.0 breaking change to simplify the code.

For each slot (title, subtitle, description, primary, props, stories):

• Remove slots
• Remove documentation
• Add migration

[shilman]

Ooh-la-la!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-alpha.30 containing PR #10259 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.