Move the Feature Reference to Read The Docs

[apauley]

Move the Feature Reference out of our README, and into
https://hledger-flow.readthedocs.io/

Also update the docURL function to point to the new URL, and ensure that existing usages will generate valid links.

hledger-flow/src/Hledger/Flow/Common.hs

Line 234 in 1729200

docURL = format ("https://github.com/apauley/hledger-flow#"%l)

[avh4]

Re help-wanted:

Is this still desired? (Imo, as a relatively new user, doing this (or using any other similar system) would be nice w/r to navigating the docs.)

I see a skeleton was deployed at https://hledger-flow.readthedocs.io/ but I don't see anything in the main branch that corresponds to that. Is the skeleton already set up (maybe on a different branch?, which I also didn't find looking at the existing branches), or does the initial setup need to be done from scratch for this?