feat: pipe buffer output to new document

[majiayu000]

Summary

• add pipe buffer actions for terminal output and for opening output as a new document
• avoid a default key binding; document how to bind pipe_buffer/pipe_buffer_doc
• add a DocPipe type to label pipe output documents

Motivation

Follow-up to feedback on #944 about offering pipe output as a new document and avoiding a default | binding.

Testing

• go test ./oviewer