feat: overhaul cmd and release process

[caarlos0]

• use cobra so its more like goreleaser
• remove gocenter from goproxy
• using "NFPM is a simple, 0-dependencies, deb, rpm and apk packager." as description in several places
• completions for bash, fish and zsh (also in the brew tap and on the packages)
• cmd docs on website
• cmd code is pretty much ready to accept unit tests (although I haven't added any yet)
• using commit timestamp et al on ldflags so the build is reproducible
• using zip for windows archives
• install the apk package inside the docker image, so we have completions et al there as well (although they wont work as we don't create completions for ash)
• create a discussion for each release

overall, mimic a good deal of config/structure from goreleaser into nfpm.

[vercel]

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[codecov]

Codecov Report

Merging #325 (a356dfe) into master (1fb2691) will decrease coverage by 2.00%.
The diff coverage is 36.31%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   69.37%   67.36%   -2.01%     
==========================================
  Files           9       14       +5     
  Lines        1159     1284     +125     
==========================================
+ Hits          804      865      +61     
- Misses        223      285      +62     
- Partials      132      134       +2     

Impacted Files	Coverage Δ
cmd/nfpm/main.go	0.00% <0.00%> (ø)
internal/cmd/docs.go	0.00% <0.00%> (ø)
internal/cmd/init.go	0.00% <0.00%> (ø)
internal/cmd/package.go	0.00% <0.00%> (ø)
internal/cmd/root.go	0.00% <0.00%> (ø)
rpm/rpm.go	73.20% <60.00%> (-0.67%)	⬇️
apk/apk.go	70.29% <100.00%> (+0.25%)	⬆️
internal/cmd/completion.go	100.00% <100.00%> (ø)
nfpm.go	86.88% <100.00%> (+1.56%)	⬆️
... and 4 more

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[djgilcrease]

LGTM

[github-actions]

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