Integrate "How to write a recognition method for the recog package" from HackMD note into documentation

[fingolfin]

Years ago some of us drafted a document "How to write a recognition method for the recog package" and put it on HackMD with the plan of integrating it in the user manual. Unfortunately that never happened. Perhaps we can do it now?

[fingolfin]

Argh I am wrong, we did integrate it. I should check first instead of trusting my bad memory

[fingolfin]

Reopening as I discovered that we did not migrate all of that HackMD, and the result is that the "How to write a recognition method" is worse off as some crucial things are not covered anywhere.

In particular, it seems the HackMD was migrated into our docs only starting with section "Leaf methods". But most things before that are not in there, at least not in the "How to..." chapter. Specifically the bit where we define "leaf" and "splitting methods (of course the plan now is to change all of those to "non-leaf method", see issue #254).

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Anyway: perhaps a first step would be simply copy the first half of the HackMD into our manual, too. There is some overlap with other parts of the manual, but I'd rather resolve that up later, than having an inscrutable HOWTO. In particular we can then forget about the HackMD, and any "refactoring" of the manual can be done locally within the recog repository.