Signatures in documents

ISO 27001 does not prescribe signatures for documents, only that documents are approved before use. Considering that, provided you can ensure that documents content, in physical or electronic format, are approved before use you can adopt any approach you see fit (e.g., signing the front page of each document, keeping electronic versions of approved documents in a specific folder, etc.).

Signing a single document for all processes like you suggested is acceptable but not recommended, because users of documents won't be able to easily check if the document they have on hand is an approved version (signing the front page, or use of specific electronic folder,  are examples which provides a balance between the need for signing and easiness of approval verification)

This material will also help you regarding document control:

• Managing ISO Documentation: A Plain English Guide https://advisera.com/books/managing-iso-documentation-plain-english-guide/