Improve readme example code

[alyti]

Based on this article that covered use of this library the following changes from it's summary could be applied to the current readme:

• mipidsi documentation should include required use statements in its example code, as well as linking out to required crates
• mipidsi documentation should not reference the no-longer-used Rgb666 color type

These changes would improve beginner journey.

[almindor]

I can have a look, one problem is that a "real" example case would need to use some sort of specific embedded-hal implementation to get the SPI for example. That would bring a whole tree of dependencies in, and would be generally useless as an example because it'd only work for people with that setup anyhow.

[almindor]

• mipidsi documentation should not reference the no-longer-used Rgb666 color type

What do you mean with this exactly? Rgb666 is a valid color, it's just RawU18 that will go away AFAIK