Add the existing Integration Video to the templates page in the docs

[MantisClone]

Definition of Done

• Add the Demo Video and Integration Video to the Templates page in the docs under headings: "Demo Video" and "Integration Video", respetively.
• Link to the "Integration Video" heading from the Create Invoice Form and Invoice Dashboard pages.

Precision

This task is not meant for recording a new integration video.

[MantisClone]

Resolved in 967e21e