Documentation on creating previews via

[stevieflow]

I note we have some documentation for people who wish to create previews through methods not set in the upload UI:

https://github.com/notshi/portald/blob/main/upload.md

Should we link to these / create a page from markdown - in terms of giving more documentation to users?

@Bjwebb may have an opinion, too

[notshi]

Have added it to https://github.com/notshi/d-preview/blob/main/README.md