List all custom assertions in submodule readme

[Inkrementator]

This allows you to click on the sub-package on code.dlang.org and get a list. This is something have now done a few times (instinctively, the affordance is definitely there for it) but I never found anything and had to go source diving for the functions.

Extracted with the following commands from the git root dir:

PATTERN='^\s*(?:[A-Za-z_@()\s]*\s)?(?:auto|void)\s+(should[A-Z][A-Za-z_0-9]*)'
FILE=subpackages/assertions/source/unit_threaded/assertions.d
rg -P --no-line-number --only-matching --replace '$1' "$PATTERN" "$FILE" \
	| awk '!seen[$0]++' \
	| sed 's/^/- /'