doc: Standardize capitalization of term "hot standby"/"Hot Standby"

doc: Standardize capitalization of term "hot standby"/"Hot Standby"

"Hot Standby" was capitalized in a couple of places in the docs, as the
style primarily used when it was introduced, but this has not been much
respected across the years.  Per discussion, it is more natural for the
reader to use "hot standby" (aka lower-case only) when in the middle of
a sentence, and "Hot standby" (aka capitalized) in a title.  This commit
adjusts all the places in the docs to be consistent with this choice,
rather than applying one style or the other midway.

Author: Daniel Westermann
Reviewed-by: Kyotaro Horiguchi, Aleksander Alekseev, Robert Treat
Discussion: https://postgr.es/m/GVAP278MB093160025A779A1A5788D0EAD2039@GVAP278MB0931.CHEP278.PROD.OUTLOOK.COM