Styling for warning/info/note

[evangelion1204]

Added styling for warning, note and info messages.

Closes #7

[aik099]

This is too red and blurry (I guess blurry happens due high contrast of the color to the other colors on a page).

Now I understand why in Confluence they've used less contrast colors.

[evangelion1204]

I changed to colors a bit and added some radius again to get a contrast not by color alone but by design.

[aik099]

I like inverted color idea and round borders, but color is still too contrast. Try changing to be as soft as ones used in existing color scheme.

[aik099]

Maybe also add test page where all changes can be seen at glance, e.g. /test_messages.md.

[evangelion1204]

Ok, there it is. The alpha can be removed later, don't worry, just makes things a bit easier on the white background.

[aik099]

Ones with warning round pastel and info round pastel class looks good.

Yellow ones look a bit off through. Needs another try.

[evangelion1204]

Ok, another next try 😄

[aik099]

Yellow too light. Maybe you should try adding more colors at once, e.g. 5 yellow variations.