Documentation: add mlr3book chapter

[sebffischer]

• What is functional data

• How do I get my data into the tf data, which data types are there and how do they work

• How does the package worrk

• Showcast the use of the PipeOps
  Other comments:

• We can take inspiration from: https://arxiv.org/pdf/1911.07511.pdf

[m-muecke]

Chapter lives in the following branch: https://github.com/mlr-org/mlr3book/tree/fda