Rewrite of the Code4z readme to point people at Techdocs

• What's New is replaced by the New Features page: https://techdocs.broadcom.com/us/en/ca-mainframe-software/devops/code4z/2-0/release-notes/new-features.html
• The sample workflows have been moved to the main "using" page: https://techdocs.broadcom.com/us/en/ca-mainframe-software/devops/code4z/2-0/using.html
• The "How Does It Work" section is now on the main "getting started" page; I left a shorter summary here. https://techdocs.broadcom.com/us/en/ca-mainframe-software/devops/code4z/2-0/getting-started.html
• The intros to each extension have been cut down to single sentences, these are challenging to maintain (some are probably out of date). The same descriptions can be found on the Release Notes page: https://techdocs.broadcom.com/us/en/ca-mainframe-software/devops/code4z/2-0/release-notes.html
• Links to each extension's page on the marketplace have been kept, users can follow those to find out more about each extension
• Next week I will go through the other links from the extension subsections and decide which ones to add to the "Additional Resources" page.
• Alt text of the image has been improved.