Make the comparison table of form selection components DRY

From !1557 (comment 250498700).

Move the comparison table for checkbox, dropdown, radio, segmented control, and toggle, that is duplicated across all of those component pages, into a separate file that is included/imported as a partial. Make it DRY!

https://www.npmjs.com/package/markdown-it-include might be a good solution since we're using markdown-it to render markdown files in Pajamas:

Markdown-it plugin which adds the ability to include markdown fragment files.