Bring history.md up to date

history.md: https://git.glasklar.is/sigsum/project/documentation/-/blob/main/history.md?ref_type=heads

Things to possibly add from the top of my head:

• CATS 2023 (filippo witness talk, nisse sigsum lightning talk), perhaps inspo in what was backed out in https://git.glasklar.is/sigsum/project/documentation/-/merge_requests/58/diffs?diff_id=5521&start_sha=2f723930efc27b78616b8a668f8776b32ad60de4 because the event had not happened yet.
• Specs donated/moved to C2SP (https://git.glasklar.is/sigsum/project/documentation/-/blob/main/proposals/2024-01-on-specifications-and-governance.md?ref_type=heads)
• C2SP release candidates have been tagged (https://github.com/C2SP/C2SP/issues/97#issuecomment-2315900369)

(Hopefully we will be able to also merge an entry about stable/prod log before transparency-dev summit.)