Doc : update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 15th of May.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last update of Alpha changelog (!8616 (merged)).

• !8033 (merged) - plugin only
• !8051 (merged)
• !8502 (merged) - tests only
• !8507 (merged) - environment only
• !8563 (merged) - already included (thanks @ACoquereau)
• !8566 (merged)
• !8579 (merged) - already included (thanks @rafoo_)
• !8622 (merged) - doc only
• !8657 (merged)

The list was generated using the following commands:

$ git log --reverse --merges --full-history 8b74239d..76234777 -- src/proto_alpha/lib_protocol/ src/lib_protocol_environment/sigs/v*/ | grep 'See merge request' | sort

Manually testing the MR

Please read the description of changes.