docs: Add acronym and glossary entries for I/O and AIO

docs: Add acronym and glossary entries for I/O and AIO

These are fairly basic, but better than nothing.  While there are several
opportunities to link to these entries, this patch does not add any. They will
however be referenced by future patches.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250326183102.92.nmisch@google.com