Training: Request Manager and its CherryPy threads

[amaltaro]

Impact of the new feature
WM in general

Is your feature request related to a problem? Please describe.
Sharing knowledge and empowering WM/Ops to understand and debug the system. Sub-task of #11837

Describe the solution you'd like
Topic of this ticket is: Request Manager and its CherryPy threads

For the expected solution, please refer to the meta-issue for further details (in short: documentation + recorded presentation).

Describe alternatives you've considered
None

Additional context
None

[anpicci]

I'm closing this issue, since @vkuznet successfully led the training session