docs: add how-to guide on integrating third-party services

[adamstankiewicz]

Description:

Taking a stab at some more formal documentation around how to integrate third-party, external services with the frontend-platform library, including current & proposed patterns.

Merge checklist:

• Consider running your code modifications in the included example app within frontend-platform. This can be done by running npm start and opening http://localhost:8080.
• Consider testing your code modifications in another local micro-frontend using local aliases configured via the module.config.js file in frontend-build.
• Verify your commit title/body conforms to the conventional commits format (e.g., fix, feat) and is appropriate for your code change. Consider whether your code is a breaking change, and modify your commit accordingly.

Post merge:

• After the build finishes for the merged commit, verify the new release has been pushed to NPM.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.41%. Comparing base (c878cce) to head (3a3556d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #725   +/-   ##
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  Coverage   83.41%   83.41%           
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  Files          40       40           
  Lines        1073     1073           
  Branches      197      197           
=======================================
  Hits          895      895           
  Misses        166      166           
  Partials       12       12           

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