added header and the index file for Template Engine
Updated API_reference index
Fixed plural URL to be singular
rewrite of section tag page.
fix invalid JSON
added ESP link
Merge pull request #786 from sendgrid/task246

added header and the index file for Template Engine
Emai to Email
Update section_tags.md

Good point, that was leftover from pre-edit.
Merge pull request #790 from SuePom/patch-6

added ESP link
Update index.html

According to the SMTPAPI validator (https://sendgrid.com/docs/API_Reference/SMTP_API/smtpapi_validator.html), the above example is invalid. Fixing it to use the expected list form.
Corrected small typo
Merge pull request #792 from anant90/patch-1

Corrected small typo
Merge pull request #788 from sendgrid/task685

Updated API_reference index
Merge pull request #789 from jacobmovingfwd/docs435-538

rewrite of section tag page.
Merge pull request #791 from nathanielweinman/patch-1

Update index.html