Adding UI extension skills

Given that UI extensibility is an important part of the AEM experience I propose that we add skills related to both Universal Editor and Content Fragment Editor extensions in this library.

The skills included should follow a typical pathway of creating a UI extension for UE and CFE. They should include the following steps:

- Planning & Analysis - plan the required work for the extension
- Scaffolding - create a minimum viable extension scaffold
- Build - build the extension code
- Configure - create any and all neccessary configuration for the extension to be successfully deployed
- Distribute - package and deploy the extension
- Troubleshoot - identify and resolve issues
- Validate & harden - make sure the extension is production ready

I've created a sample library in a fork but wanted to open this up for discussion before raising a PR.

Review the sample library in the author's fork first, then compare its Universal Editor and Content Fragment Editor skill structure with the repository's existing skills. Done means the library includes the proposed pathways—planning, scaffolding, build, configure, distribute, troubleshoot, and validate/harden—for both extension types.

Written by the indexing model from the issue text.