Improve Notary Project signature specification

[yizha1]

1. Remove Extended attributes for Notation Plugins
   Our goal should be to have portable signatures and not tie them to a specific tool. Those two extended attributes are very specific to a single implementor, and thus they should be in the signature spec. We can consider adding those attributes to plugin specifications
2. Clarify "See Guidelines for implementations of the Notary Project signature specification for options to handle these attributes during signature verification.", since the link pointed to the specification itself
3. Timestampingis not a thing yet. Therefore, all specs related to timestamping should be marked as drafts.
4. Providing examples of the signatures and the certificates as part of the spec or at least in the Guidelines for implementation would be very beneficial for readers to understand what needs to be done.
5. Expiry sounds a bit odd name for a section

[yizha1]

Close as it is outdated