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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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