Add ZendMM internals, lifecycle and how to hook into docs

[realFlowControl]

Hey there 👋

this PR will add a missing documentation about the ZendMM internal design, as well as a section about the lifecycle of the Zend Memory Manager and an example of how to hook into it.

[Girgias]

@arnaud-lb and @iluuu1994 you may know more than I do about ZendMM.

[arnaud-lb]

Looks good to me, thank you!