We have created more HTL (used to be named Sightly) articles based on community feedback like this. Here is one on SOAP Web Services:
1 - https://helpx.adobe.com/experience-manager/using/webservice_htl.html
Here is one on using AJAX from an HTL component:
http://scottsdigitalcommunity.blogspot.ca/2016/08/creating-aem-html-template-language_16.html
As you can see, HTL is only HTML tags - it does not call a RESTFul or SOAP web service. That is done on the HTL Java classes - as shown in the 1 article. THen results are returned and displayed in the HTL component.
We will write a HTL and RestFul Web Service article soon, It will be similar to 1 but instead of consuming a SOAP web service, it will consume a Restful web service.
Also - we will create a video for these articles too.