Update shortcodes.md

[Jieiku]

Zola no longer bundles shortcodes, they must be added to themes as needed.

Currently you cannot clone this repo and zola build with the latest version of zola.

It errors out because of the missing shortcodes.

This pull request allows the theme to build again.

[c-git]

I've tested the changes in this PR and verify that it does not build before the change and does afterwards.

[codeandmedia]

thank you!