Code Splitting components and exposing endpoints

This is required for make product consumers able to load the components asynchronously. In that case it's following similar process as we already using in packages suck as @atlaskit/icon. To make it work, I created a folder called entrypoints pointing to the bundled files.

To make it possible, this pull request:

• Adds docs for Entrypoint logic/approach

• Creates a build npm script in a package level

• Creates entrypoints folder to make components available without break/tight consumers and package bundle code;

• Adding theming logic by type into Skeleton component to be used on Async components on JFE

• Adding docs for code splitting and entrypoints

• Exposing SkeletonContainer component

• Accepting Skeleton and viewRenderer as props;

• Adding Storybook examples using react-async-component

This is the first step of the code-splitting task for nav-next. More details about this approach in this link:

https://product-fabric.atlassian.net/wiki/spaces/NAV/pages/811696794/Navigation+next+code+splitting+-+Hypothesis+approaches+and+improvement+strategy