Update release notes and version for 3.0.0a1

[EwoutH]

Update release notes and version for 3.0.0a1. Also includes the 2.3.2 release notes.

Draft release is prepared here.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -0.5% [-0.8%, -0.1%]	🔵 -0.0% [-0.2%, +0.2%]
Schelling	large	🔵 -0.5% [-1.1%, +0.2%]	🔵 -0.2% [-1.0%, +0.5%]
WolfSheep	small	🔵 -0.5% [-1.6%, +0.7%]	🔵 +0.4% [+0.1%, +0.7%]
WolfSheep	large	🔵 -0.1% [-0.4%, +0.2%]	🔵 -0.2% [-0.8%, +0.4%]
BoidFlockers	small	🔵 -0.4% [-1.2%, +0.4%]	🔵 +0.1% [-0.6%, +0.7%]
BoidFlockers	large	🔵 -0.7% [-1.4%, +0.0%]	🔵 -0.4% [-1.0%, +0.2%]

[rht]

@tpike3 @wang-boyu reminder to do the equivalent of this for Mesa-Geo.