Add Button spec and examples.

Signed-off-by: Lee <jessy1092@gmail.com>
Add docs.
- Divider, Flag, Header, Icon, Image docs.

Signed-off-by: Lee <jessy1092@gmail.com>
Fix Image elements bug.
- Use <img> instead of <div>

Signed-off-by: Lee <jessy1092@gmail.com>
Add type attribute in Label elements.
- Add docs: Input, Label.

Signed-off-by: Lee <jessy1092@gmail.com>