Doc: Update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 21st of November.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last update of Alpha changelog (!6830 (merged)).

• !6047 (merged)
• !6108 (merged)
• !6414 (merged) - tests only
• !6424 (merged) - tests only
• !6468 (merged)
• !6507 (merged)
• !6676 (merged)
• !6677 (merged) - tests only
• !6698 (merged)
• !6716 (merged)
• !6811 (merged)
• !6821 (merged)
• !6844 (merged)
• !6852 (merged)
• !6860 (merged)
• !6879 (merged)
• !6884 (merged)
• !6890 (merged) - docs only

The list was generated using the following commands:

$ git log --reverse --merges --full-history cc71dd..7a316d -- 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.