minor #5369 Remove the Propel book chapter and explain why we do that (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Remove the Propel book chapter and explain why we do that

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

Commits
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3487157 Fixed issues reported by @xabbuh
b881bea Remove the Propel book chapter and explain why we do that

Author: weaverryan