Upload corpus documentation in form

[lukavdplas]

Corpora can include documentation pages, this should be added to the corpus form. Add a separate step for this in the form.

The content of each page is written in markdown, see the internal documentation.

Maybe use a ready-made component to write the markdown files, e.g. codemirror?

The backend API supports uploading the documentation contents, so this just needs a frontend component.

[lukavdplas]

Uploading is now possible, but editor needs to be improved; would like to use a WYSIWYG editor.