Add the specification state to the title page

Each specification can be in one of the following states:
Development
Stable
Frozen
Ratified

In order to avoid confusion around which specification version should be
implemented we add the current state as well as a description of that
state to the title page.

More info here: https://riscv.org/spec-state



Signed-off-by: Stephano Cetola <scetola@linuxfoundation.org>