[Docs] introduce "make help"

[6543]

as title

[sapk]

What about a more generic way like https://marmelab.com/blog/2016/02/29/auto-documented-makefile.html or https://stackoverflow.com/a/35730928 ?

[codecov-io]

Codecov Report

Merging #9604 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9604      +/-   ##
==========================================
- Coverage   42.18%   42.16%   -0.03%     
==========================================
  Files         578      578              
  Lines       76066    76066              
==========================================
- Hits        32092    32076      -16     
- Misses      39991    40010      +19     
+ Partials     3983     3980       -3

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
modules/task/migrate.go	25% <0%> (-3.95%)	⬇️
modules/process/manager.go	74.69% <0%> (-3.62%)	⬇️
services/pull/temp_repo.go	31.62% <0%> (-2.57%)	⬇️
models/error.go	32.71% <0%> (-1.11%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
routers/repo/view.go	38.59% <0%> (-0.88%)	⬇️
models/repo.go	47.97% <0%> (-0.11%)	⬇️
modules/migrations/gitea.go	9.66% <0%> (+0.63%)	⬆️
modules/migrations/migrate.go	25.78% <0%> (+1.57%)	⬆️
... and 1 more

---

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[6543]

@techknowlogick done

[6543]

@techknowlogick you still block this - are you ok with this?