DOC:Remove hard-coded examples from _flex_doc_SERIES (#24589)
Initial work on #24589 

- Removes hard-coded examples from _flex_doc_SERIES
- Adds separate examples for each op (_*_example_SERIES). At this stage I've just copied the examples which were in the _flex_doc_SERIES template (add, sub, mul, div)
- Modifies _make_flex_doc to format the _flex_doc_SERIES template with an example string from op_desc['series_examples']
- adds references to each of the examples to _op_descriptions['series_examples'], allowing them to be picked up in _make_flex_doc
 
Ops outside not in [add, sub, mul, div] will return their docstring with no examples in this revision.
