Docs: new ownership for several ESPs

For ESPs that have incorporated new owners
in their branding, identify the new owner
in the docs:

- Mailgun -> Sinch Mailgun
- Postmark -> ActiveCampaign Postmark
- SendGrid -> Twilio SendGrid
- SparkPost -> Bird ???

(Bird's rebranding of SparkPost seems to still
be a work in progress at this point. In fact,
Bird's rebranding of itself from MessageBird
seems incomplete.)

There are no current plans to rename ESP
backends, as the new owners seem to be mostly
keeping the original ESP names and domains for
API endpoints, docs, etc.

(Similarly, I'm not updating project keywords.)

README.rst

@@ -29,14 +29,14 @@ Anymail currently supports these ESPs:
 * **Amazon SES**
 * **Brevo** (formerly SendinBlue)
 * **MailerSend**
-* **Mailgun**
+* **Mailgun** (Sinch transactional email)
 * **Mailjet**
-* **Mandrill** (MailChimp transactional)
+* **Mandrill** (MailChimp transactional email)
 * **Postal** (self-hosted ESP)
-* **Postmark**
+* **Postmark** (ActiveCampaign transactional email)
 * **Resend**
-* **SendGrid**
-* **SparkPost**
+* **SendGrid** (Twilio transactional email)
+* **SparkPost** (Bird transactional email)
 * **Unisender Go**
 
 Anymail includes: