[Feature Request] Add thumbnail URL to workflow placeholders

[SingingDwarf]

Description

I would like to see the thumbnail URL added to workflow placeholders.

This is currently available via the environment variables for post consumption scripts, but is not currently available for use via the built-in workflow for use for example within the webhook.

https://docs.paperless-ngx.com/usage/?h=webhook#workflow-placeholders

Use case: Generate a Home Assistant notification, including thumbnail, when a document is processed from the email integration.

NB I appreciate that I could script this, but would prefer to use the built-in workflow if possbile.

Other

No response