Add vignette for smaller helper functions

[alkaZeltser]

This package has a lot of functions internal to other functions that can still be used individually by the users, but their potential use-cases are not as well documented in vignette format.

For example:

• convert.alleles.to.pgs.dosage
• convert.allele.frequency.to.dosage
• format.chromosome.notation
• get.pgs.percentiles

.. and several more

Would be nice to create a separate vignette for these.