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
`#CommentingInTheWrongOrder` since this is html, you can't use the markdown link syntax. [See my note below](https://github.com/sendgrid/docs/pull/593/files#r18987721).