Update REST API Providers for transactional email services

[wxactly]

Closes #

Summary

CMS Email Service on Pantheon - Update REST API Providers section, fix service names and links

Effect

The following changes are already committed:

• Update Mandrill to Mailchimp Transactional. Reason: this service has been renamed and there is a new Drupal module.
• Remove link to Mandrill WordPress plugin. Reason: this plugin has been closed and is not available for download because of a security issue.
• Remove link to SendGrid WordPress plugin. Reason: this plugin does not provide REST API support, only SMTP.
• Update Sendinblue to Brevo. Reason: this service has been renamed and there is a new Drupal module.
• Re-alphabetize the new list.

Remaining Work and Prerequisites

The following changes still need to be completed:

• [ ] List any outstanding work here

Dependencies and Timing

• [ ] Other prerequisites that must be completed before merging this PR

Release:

• [ ] When ready
• [ ] After date: $DATE

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Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)
• [ ] For Heroes - add a props post to the discussion board.
• [ ] Remove from the project board