Convert styleguide to mdx and add section on export *

[stacey-gammon]

A simple md -> mdx, and one extra section:

Avoid export * in top level index.ts files

The exports in common/index.ts, public/index.ts and server/index.ts dictate a plugin's public API. The public API should be carefully controlled, and using export * makes it very easy for a developer working on internal changes to export a new public API unintentionally.

// good
export { foo } from 'foo';
export { child } from './child';
// bad
export * from 'foo/child';
export * from '../foo/child';

I will hopefully get this into the docs system, but having trouble with it picking up the file (see https://github.com/elastic/elastic-docs/pull/323)

[kibanamachine]

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[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.x

This backport PR will be merged automatically after passing CI.