Doc : update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 6th of March.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last update of Alpha changelog (!7741 (merged)).

• !7609 (merged)
• !7729 (merged)
• !7737 (merged)
• !7746 (merged) - tests only
• !7776 (merged) - tests only
• !7779 (merged)
• !7814 (merged)
• !7828 (merged)
• !7829 (merged)
• !7838 (merged) - tests only
• !7839 (merged) - tests only
• !7848 (merged) - tests only

The list was generated using the following commands:

$ git log --reverse --merges --full-history 64e0f91b..f9675b19 -- 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.