How did -hood evolve into the noun-forming suffix commonly used in words such as childhood, priesthood, or neighborhood— and including certain pseudonyms such as robinhood which could easily be construed as a brotherhood of robbers?
It comes from -hād in Old English, which means "state or condition".
forming nouns of condition or quality, from nouns or adjectives
A condition or state of being the thing or being in the role denoted by the word it is suffixed to, usually a noun.
child - childhood
A group sharing a specified condition or state. brother - brotherhood
neighbor - neighborhood
Etymonline sees the relationship with German -heit - compare childhood and Kindheit. This suffix can be found in similar form in other Germanic languages as the wiktionary link -hood shows.
Etymonline gives as last source a PIE (s)kai bright shining. Personally I think this is a wrong track and a word family that has nothing to do with -hood or -heit as in childhood/ Kindheit where the suffix simply means state. For me it is enough to see a relationship with the Latin word class formed with the suffix -tas/tat-is or -itas/itatis as liber, adjective, meaning free and the noun libertas/libertatis meaning the state of being free, liberty or freedom. We may assume that this suffix is derived from Latin status/u:s meaning state/condition.
This connection is sufficient for me, I have no need of a PIE root as skai/kai bright/shining that explains absolutely nothing and is most probably a wrong track.
I assume that Latin -tat was changed into -hat with German -heit and English -hood.