DOCS: add mention of swagger api reference

[borisovano]

swagger api link mentioned vaguely in the FAQ but IMHO missing from API usage page.

[codecov-io]

Codecov Report

Merging #8452 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #8452      +/-   ##
=========================================
- Coverage   41.81%   41.8%   -0.02%     
=========================================
  Files         497     497              
  Lines       65639   65639              
=========================================
- Hits        27450   27441       -9     
- Misses      34674   34681       +7     
- Partials     3515    3517       +2

Impacted Files	Coverage Δ
modules/sync/unique_queue.go	78.57% <0%> (-10.72%)	⬇️
modules/avatar/avatar.go	48% <0%> (-6%)	⬇️
models/unit.go	62.16% <0%> (-5.41%)	⬇️
models/gpg_key.go	55.83% <0%> (-0.84%)	⬇️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️

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