Improve extension docs, remove deprecated rb_cData [ci skip]

rb_cData is deprecated and the characteristic alloc_func was already
removed in ruby-3.0. So this updates the recommendation accordingly.

It also adds fdbm_alloc() in order to show the allocation process
and to gives TypedData_Make_Struct() more context.

Moreover it describes fdbm_aref(), so that the relation to
rb_define_method() is shown.

And fdbm_aref() makes use of GetDBM() now, to show how this macro
might be used.

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