docs: add apoe haplotype example

[gymreklab]

Adding test VCF and haplotypes for apoe4

Example command:

haptools transform -s NA12878 tests/data/apoe.vcf.gz tests/data/apoe4.hap

[aryarm]

nice! thanks for trying it out and double checking that it works!

I think this would be a great addition to the examples section in the docs for the transform subcommand:

Examples
~~~~~~~~
.. code-block:: bash

	haptools transform -o output.vcf.gz -s NA12878 tests/data/apoe.vcf.gz tests/data/apoe4.hap

and then you could probably even retitle this PR as a documentation-based PR, for example:

docs: add apoe haplotype example

[gymreklab]

Thanks for these suggestions! I implemented these and am merging.