R package documentation: add a keras + tfio example

[missaugustina]

This stems off issue #26

"An example to start with would be using it together with keras R package and tfdatasets package. Examples for different types of datasets would be useful too."

Tasks for this issue to be resolved:

• PR w/ documentation of using tfio together with keras in a general use case
• identify & create issues for additional examples content

[missaugustina]

Please assign to me, I will start work on it once the pkgdown changes have merged.