Documentation about ssd1306 HOT 4 CLOSED

Cf. #49

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Relevant discussion in #7

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Some specific things I've noticed in the rendered docs:

• There should be a platform-agnostic example on the docs landing page with a graphics example, probably exercising all bits of the embedded_graphics API (triangle, square, circle, text, image). This will get updated with a following char mode example when #27 is ready. A link to a runnable example demonstrating the same for each display mode would be good too.
• To extend the previous point, there should be an explanation of the concept of display modes; this driver can't be used without choosing one!
• There should be better links between types/other language constructs in the docs
• Readme:
  • The image needs to go on a diet. It's huge at the moment.
  • graphics is on by default and no longer optional so should be de-listed
  • A complete copypasta for stm32f103 would be good
  • Link to examples directory
• The re-exports list needs an audit. This is related to #5 so can probably be fixed over there
• Should there be a description of how all the modules fit together?
• builder needs a couple of examples. I suggest:
  • A default/basic one that just sets up a display in graphics mode with SPI
  • One that exercises as many of the builder options as possible that changes the display size, rotation and I2C address in I2C mode.
• Rotate180 doesn't actually flip the display, it just (uh...) rotates it.
• interface should mention the ridiculous and not so ridiculous types required by RTFM apps.
• Each mode should have an explanation of what it does. Also worth mentioning that RawMode shouldn't be directly used and is only a helper for the builder.
• borrow_iface_mut could use a short usage example. Method was removed in #49.
• Make sure each module has a more-than-one-line description of what it does, preferably with a code example.

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borrow_iface_mut could use a short usage example.

Actually that should go away and the "DisplayProperties" (or whatever the name will end up to be in the end) should provide proper methods to change configuration and send data.

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