Integrate Summon to Discovery Showcase

[amusick-endava]

• Refactored existing discovery showcase such that search-carousel and document-card contain no parsing logic, dummy components that display parameterized data, and moved logic and parsing to pnx-search-carousel and pnx-service while moving shared logic to image-service
• Added snx-search-carousel and snx-service for use by summon
• Added snx-search-carousel to the index.html demo
• rollup command now builds two separate bundles, one for pnx and another for snx
• edited readme accordingly

NOTE:::
The risk here is around how we have changed names of components. For example pnx-card is now document-card. This would break styles for someone who wrote them expecting pnx-card but downloaded a new bundle and would require a change in documentation. https://developers.exlibrisgroup.com/blog/primo-showcase-how-to-embed/

If we want to go forward with adding Summon but be more cautious we can undo any renaming.