Restructure and shorten keyboard section

[mcking65]

The section Developing a Keyboard Interface is very long and mixes rationale and "how to". It does not effectively serve readers who need quick "how to" guidance.

Restructure the section to address this issue.

For example, consider making a short section that walks the reader through a series of steps for designing a keyboard interface. This section could include links to other sections that provide detailed descriptions of important concepts. The detailed description sections are also important because they are link targets from design patterns.

The detailed description sections can also be made shorter by linking to illustrative example pages.