Remove unused theme content

[ionous]

re: https://github.com/shift-org/shift-docs/issues/569

complete details in the commit list, basically:

• removes unused content from site/data, theme/layouts, theme/static, and theme demo images
• related to that content: removes some javascript from static/js/front.js, and some site variables from config.toml
• adds site/readme with details on the various directories ( and merged in some of docs\theme_editing.md )

this also fixes https://github.com/shift-org/shift-docs/issues/587. those warnings were all from outdated theme content.