Update Send SMS section of developer docs and README

[PeterOcansey]

Let's update our our developer documentation and README for the SMS section to accurately represent the SDK's current implementation.

Acceptance criteria

• [x] Add SMS to table of content of README
• [x] Update .setRecipients to .setTo at send sms section
• [x] Check and update import

Further discussions here: https://themailergroup.slack.com/archives/C01G23Y28NL/p1681719147825509