Create a new page for collapsible composite field in Magnolia 6 UI.

1. Create a page titled Collapsible composite field - 6 UI as a child of Magnolia 6 UI documentation. NOTE: this page could actually go under Composite field - 6 UI (see MGNLUI-5386) since it is a type of composite field. It might not need an entry at the same level as Composite field - 6 UI.
2. Copy https://documentation.magnolia-cms.com/display/DOCS62/Collapsible+composite+field in its entirety to the new page.
3. Rewrite/update the newly created page in DOCS62 to reflect the Magnolia 6 UI framework.

NOTE:

• YAML definitions, lists of properties, and fully qualified class names are probably the items most affected with the recent changes.
• For UI definitions, we do not include both YAML files and JCR node trees anymore. This practice was done away with starting from DOCS61. The YAML definition now suffices.
• The most basic UI definition pages have three main sections: a brief description at the beginning, an example YAML definition and a list of properties.
• The focus should always be on getting the content out. Screenshots, elaborate descriptions, etc. can wait for a later iteration when the time allows.

For inspiration, see:

https://documentation.magnolia-cms.com/display/DOCS62/Dialog+definition+-+6+UI

https://documentation.magnolia-cms.com/display/DOCS62/Form+definition+-+6+UI

https://documentation.magnolia-cms.com/display/DOCS62/Field+definition+-+6+UI

https://documentation.magnolia-cms.com/display/DOCS62/Link+field+-+6+UI