Policy and Supporting Documents: viewable attachments and external links/references

### Problem description
In some cases policy workflow requires user access to (or display of) documents/images - e.g. for T&Cs display. Guardian policy definition language currently lacks tools to enable policy authors to include such functionality into the policy.

### Requirements
Introduce the capability of add external references and artifacts to the policy at the authoring time. At least the following formats/methods must be supported (all have description/link):

1. Attach files (pdfs, images) - to be embedded with the policy at publishing time
2. URLs (http/https)
3. CIDs (IPFS content links)

At the policy execution time, whenever such items are encountered, the form must natively display the description of the item and the link to the artifact in a way that makes it convenient for users to view it.

### Definition of done

- Functionality is implemented as described above
- Documentation has been updated to provide users with instructions on how to use the feature

### Acceptance criteria
1. Policy authors can included additional material into the policy as embedded content or as links to external resources.
2. The material is then conveniently accessible to users at policy run time