…#12707

This fixes a bug where previously the `category_key` -- an optional
grouping that can be specified on glossary terms -- was used directly
as the group name to place terms within in the generated HTML index.

The fix re-uses a similar logic flow as used in `_key_func_1`, where
the `category_key` is used to derive (but not as an absolute source
of) the index group.

The grouping key (`category_key`) from glossary terms is used to
decide what symbol/character to group the corresponding terms into.

However: the terms are not guaranteed to begin with the same symbol
as the `category_key`.  This code therefore places them into a nested
definition entry in the index, with the complete `category_key` as
the parent label.

Relates to commit 4255bbe.

Relates-to commit 295ee97.