📝(documentation) Add custom export templates section with usage instructions and limitations

[rvveber]

Purpose

Documentation about how adding custom templates was lacking.

[rvveber]

Indeed it is a lacking documentation. I think we should wait a bit to merge this PR because we want to refactorise this template system, we have these 2 issues about it:

• #1222
• #680

I would say that the best way to do template for the moment is like that:

Feature.multi.columns.-.Docs.online-video-cutter.com.mp4

Hello, sure. The video is already half of the documentation. Whats left is just copy as HTML and paste into backend template.