The YAML strip block chomp operator causes all trailing newlines to
be stripped from the map definition.

The previous make/shell based approach results in only a single
placeholder id 'placeholder-0' because we invoke generate_card.py
for each card which will initialize the placeholder id count each time
with 0.
Now we generate the whole car listing in python which greatly reduces
the number of program invocations and allows the placeholder id
to be incremented for each listed card.

* move more generic code handling data to data.py
* fix markdown generation in generate_deck.py
* make gen_hoverable_link more flexible by introducing callbacks
  to generate the relevant link components.
* fix the link targets for hoverable links in the generated card markdown

Mypy >= 0.780 complained about finding a source file multiple times once
via the passed path and once in the module scripts.