CSS Customization: Add Example for changing Avatar position

[alexteusz]

Hey @kwinto and @pedily ,

Could we please add the following example to the related CSS class documentation in css-customization.md in order to show that one could change the bot avatar position from left bottom to left top?

[data-cognigy-webchat-root] [data-cognigy-webchat].webchat .webchat-message-row.bot {
  align-items: flex-start !important;
}