Doc: Update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 17th of October.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v{6,7,8}/ since the last update of Alpha changelog (!6477 (merged)).

• !6364 (merged)
• !6421 (merged) - tests only
• !6350 (merged) - included in Lima changelog
• !6388 (merged)
• !6413 (merged)
• !6519 (merged) - included in Lima changelog
• !6527 (merged)

---

• !6526 (merged)
• !6505 (merged)
• !6545 (merged)
• !6508 (merged) - docs and tests only
• !6564 (merged)
• !6513 (merged)
• !6543 (merged)
• !6434 (merged) - tests only

The list was generated using the following commands:

$ git log --reverse --merges --full-history d4d547..708c98 -- src/proto_alpha/lib_protocol/ src/lib_protocol_environment/sigs/v{6,7,8}/ | grep 'See merge request'

$ git log --reverse --merges --full-history 708c98..247bb4 -- src/proto_alpha/lib_protocol/ src/lib_protocol_environment/sigs/v{6,7,8}/ | grep 'See merge request'

Manually testing the MR

Please read the description of changes.