WIP doc: integrate howto write a recog method

[ssiccha]

I'm putting this here for now so that it doesn't get lost.

TODO: Update this wrt the new RecogMethod.

[codecov]

Codecov Report

Merging #248 (7ec76e3) into master (c0bc1c3) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 7ec76e3 differs from pull request most recent head e622ae9. Consider uploading reports for the commit e622ae9 to get more accurate results

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage   78.50%   78.48%   -0.03%     
==========================================
  Files          37       37              
  Lines       17375    17375              
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- Hits        13640    13636       -4     
- Misses       3735     3739       +4     

Impacted Files	Coverage Δ
gap/perm/giant.gi	90.54% <0.00%> (-0.91%)	⬇️

[fingolfin]

I am confused why this wasn't part of PR #250 and thus part of the HOWTO that was merged.

Yes, some of this could also be used to improve other parts of the manual. But the point here was to have it in the HOWTO specifically so that people have all the information immediately available, instead of having to first read an unknown set (possibly all) of the other chapters/sections of the manual...