Clarify `kontrol build` behavior

[palinatolmach]

Identified during the Secureum workshop. We should clarify the following behavior of kontrol build in the documentation

• if Solidity files changed, we detect it by comparing digests that we generate during each kompilation, and, if the digest changed, trigger rekompilation (so in that case kontrol build --rekompile and kontrol build are equivalent, and that is why you see a change in the verification result); that is done here: https://github.com/runtimeverification/kontrol/blob/7ee1b0473aa177f95440876c21154d2d944248e2/src/kontrol/kompile.py#L139
• if Solidity files did not change, kompilation will be skipped
• in this case, --rekompile is needed if you want to add something else to the kompilation, in particular, lemmas — in other words, if you want to retrigger kompilation even if there're no changes in the forge build output that would trigger rekompilation automatically.

[palinatolmach]

Closed in favor of #54.