docs: document initial analysis on autofit feature
acpt: extract txt-textframe-props.feature

to segregate features related to TextFrame as distinct from Paragraph,
etc.

* make some naming convention updates along the way
acpt: add scenario for TextFrame.auto_size getter
txt: add TextFrame.auto_size getter
acpt: add scenario for TextFrame.auto_size setter
txt: add TextFrame.auto_size setter
docs: update docs for TextFrame.auto_size

* extract enumeration pages to user/enum/ directory