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.