Documentation improvements

[danielrbrowne]

This PR:

• Improves the script for generating API documentation
• Adds Apple Quick Help documentation comments to cover undocumented public API
  • Also updates the API docs pages based on these changes
• Adds some badges to the README (for consistency with the other Swift libraries)

[codecov-commenter]

Codecov Report

Merging #172 (3f60eda) into master (d4eaf82) will decrease coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
- Coverage   90.24%   90.23%   -0.01%     
==========================================
  Files          58       58              
  Lines        2972     2981       +9     
==========================================
+ Hits         2682     2690       +8     
- Misses        290      291       +1     

Impacted Files	Coverage Δ
Sources/Models/AuthData.swift	100.00% <ø> (ø)
Sources/PushNotifications.swift	87.80% <ø> (ø)
Sources/PushNotificationsStatic.swift	40.21% <ø> (ø)
Sources/Services/BeamsTokenProvider.swift	84.84% <ø> (ø)
Sources/Protocols/ReportEventType.swift	33.33% <0.00%> (-57.15%)	:arrow_down:
Sources/Extensions/URL+NetworkService.swift	80.00% <0.00%> (-20.00%)	:arrow_down:
Sources/Models/ServerSyncJob.swift	69.07% <0.00%> (-9.28%)	:arrow_down:
Sources/Services/NetworkService.swift	70.00% <0.00%> (-5.63%)	:arrow_down:
Sources/Services/ServerSyncProcessHandler.swift	82.21% <0.00%> (-0.40%)	:arrow_down:
Sources/Services/EventTypeHandler.swift	64.86% <0.00%> (+43.24%)	:arrow_up:
... and 1 more

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