You do not have the rights to ... redistribute (even for free) the template
The right to copy the software is one of the four freedoms of free software, so that template is non-free. Moreover, the licence is explicit that you may not sublicense the template.
As regards using the template in your website, this seems to be permissible, provided that (as you say) you maintain the prescribed footer text. You have contributed to the website, so adding your own copyright line is definitely in order. But when it comes to open-sourcing the work on GitHub, I don't see that you have the rights to copy the template to GitHub in the first place, and you definitely don't have the right to redistribute it under GPLv3.
I can't say how intimately tied-up in your work this template is. If it's trivially-removable, then you can redistribute your work under GPLv3 (via GitHub, if you please) as long as you leave the template out of it, perhaps replacing it with a notice that a copy of PrettyDocs is required to make it work, and a pointer to where the user can get one. If what you've done makes your work a derivative, in copyright terms, of PrettyDocs, then your work is all compromised, and you cannot redistribute it in this way.
If there is a useful lesson to be learned here, it may be that next time you start a work with the intent of making it free software, you should found it on free software, not merely zero-cost software.
