Refactor sassdoc task to remove Gulp

[domoscargin]

What

Update tasks/gulp/sassdoc.js to replace Gulp with our own sassdoc build script.

Why

We have made the decision to move away from Gulp and write our own build scripts.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• [ ] tasks/gulp/sassdoc.js no longer depends on Gulp
• [ ] sassdoc dependency is up to date
• [ ] sassdoc.js has been relocated to tasks folder