Cleanup SMTP API Overview

- Added code block specific header
- Make ""to:"" and other header names bold
- Add a link to limitations on the ""to"" section or reference the 10,000 email limit
- Remove this statement "When setting filters via SMTPAPI, all filter names and setting names must be lowercase."  -- it will be moved to Filters page
- MAIL FROM - give an example or link to a place that explains this more fully
- Remove "For substitution tags, it is best to avoid characters that have special meaning in html, such as <,>, and &. These might end up encoded and will not be properly substituted." --will be moved to the substitutions section
This is an html doc, so you can't use the markdown link format, you'll need to change 

```md
[Bounce Forwarding](https://support.sendgrid.com/hc/en-us/articles/200181478-How-to-set-up-bounce-forwarding-to-go-to-the-email-s-FROM-address)
```

To

```html
<a href="https://support.sendgrid.com/hc/en-us/articles/200181478-How-to-set-up-bounce-forwarding-to-go-to-the-email-s-FROM-address">Bounce Forwarding</a>
```