Doc : update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 1st of May.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last reset of Alpha changelog (!8474 (merged)).

• !5800 (merged)
• !7373 (merged)
• !8128 (merged) - already included
• !8208 (merged)
• !8276 (merged) - already included
• !8349 (merged) - migration from Nairobi to Alpha
• !8400 (merged) - tests only
• !8403 (merged)
• !8412 (merged) - doc only
• !8420 (merged)
• !8528 (merged) - already included
• !8529 (merged) - build system only
• !8546 (merged)
• !8550 (merged)
• !8562 (merged)

The list was generated using the following commands:

$ git log --reverse --merges --full-history f2eabc0d..8b74239d -- 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.