Reorganize asset documentation.

Signed-off-by: crynobone <crynobone@gmail.com>
Remove redundant url.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge branch '2.1' into 2.2

Signed-off-by: crynobone <crynobone@gmail.com>

Conflicts:
	src/components/asset/index.md
Add messages component.

Signed-off-by: crynobone <crynobone@gmail.com>