docs: adding baseurl changes for regional email to the api specs

[tbischel]

Fixes

SendGrid docs reference api.sendgrid.com as the URL for running the API against. Moving forward, this will include api.eu.sendgrid.com for regionally pinned subusers. This updates the docs for products available in the EU to point to the correct URL source.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [ ] I have read the Contribution Guidelines and my PR follows them
• [ ] I have titled the PR appropriately
• [ ] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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