Final touches before new major release

[fpseverino]

• Improve and add missing documentation
• Refactor helper structures to be nested inside main structures
• Make MailSettings.Setting and MailSettings.Footer conform to ExpressibleByBooleanLiteral

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e165ef0) to head (775e47d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          141       145    +4     
=========================================
+ Hits           141       145    +4     

Files with missing lines	Coverage Δ
...endGridKit/Models/AdvancedSuppressionManager.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/EmailAddress.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/EmailAttachment.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/EmailContent.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/MailSettings.swift	100.00% <100.00%> (ø)
Sources/SendGridKit/Models/Personalization.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/SendGridEmail.swift	100.00% <ø> (ø)
Sources/SendGridKit/Models/TrackingSettings.swift	100.00% <100.00%> (ø)
Sources/SendGridKit/SendGridClient.swift	100.00% <100.00%> (ø)