[API] Add better Swagger description and a example for GetArchive

[6543]

close #12741

[codecov-commenter]

Codecov Report

Merging #12743 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #12743      +/-   ##
==========================================
- Coverage   43.40%   43.39%   -0.02%     
==========================================
  Files         648      648              
  Lines       71581    71581              
==========================================
- Hits        31072    31063       -9     
- Misses      35482    35486       +4     
- Partials     5027     5032       +5     

Impacted Files	Coverage Δ
modules/migrations/gitlab.go	1.04% <0.00%> (ø)
routers/api/v1/repo/file.go	64.00% <ø> (ø)
modules/notification/mail/mail.go	34.48% <0.00%> (-3.45%)	⬇️
modules/notification/ui/ui.go	80.55% <0.00%> (-2.78%)	⬇️
services/pull/check.go	47.69% <0.00%> (-2.31%)	⬇️
models/notification.go	66.13% <0.00%> (-0.92%)	⬇️
models/error.go	34.81% <0.00%> (-0.86%)	⬇️
routers/repo/view.go	37.98% <0.00%> (+0.64%)	⬆️
modules/git/blame.go	77.19% <0.00%> (+1.75%)	⬆️
models/unit.go	49.31% <0.00%> (+2.73%)	⬆️
... and 1 more

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