Add note to Building from Source section about building with Spicy

[timwoj]

As a user brought up in zeek/zeek#3064, building with spicy included is challenging on systems with low amounts of available memory. We should probably add a note to the https://docs.zeek.org/en/current/install.html#building-from-source section that calls out this constraint. We typically recommend somewhere around 2-3GB/CPU job for Zeek to build correctly with Spicy included.