DOC: add Blatt–Weisskopf with Hankel report

[redeboer]

Implementation proposal for ComPWA/ampform#417.

Tip

See preview of TR-029 here.

[mmikhasenko]

Very good job.
I suggest adding a citation to the original paper by Hippel and Quigg

The paper suggest using them as FF, and list a few lower polynomials,

Maybe worth mentioning difference of small h and big H.
https://mathworld.wolfram.com/SphericalHankelFunctionoftheFirstKind.html

[redeboer]

Thanks a lot, also for the difference between spherical and 'general' Hankel functions!
Von Hippel and Quigg were cited, but now more clearly :)

[shenvitor]

Approved 🚀