Feat: Add template-expressions reference to campaign content page

[c-seeger]

Is your feature request related to a problem? Please describe.

When creating a new campaign and adding some content it would be nice to have the direct shortcode references at hand.

Describe the solution you'd like
Add shortcode references to the content page of a campaign by either adding an info button containing all shortcodes or a link to the documentation e.g https://listmonk.app/docs/templating/#template-expressions