[basic.align], [obj.lifetime], [new.delete] Clarify uses of "suitably aligned"

[Eisenwave]

As discussed in #7843, there are certain uses of "suitably X" which aren't as clear as they should be. We use "suitably aligned" in a number of places, but that is not a formal term, and it is not in the index.

To clarify these uses of the term, this PR adds an \indextext{suitably aligned}, adds references to [basic.align] in [obj.lifetime], and rewords parts of [new.delete].

The last of these is the biggest change. The original wording is

Return a non-null pointer to suitably aligned storage\iref{basic.stc.dynamic}

This is confusing because it looks like "suitably aligned storage" is a defined term which would be found in [basic.stdc.dynamic]. In reality, it's not a defined term, and there are merely some rules more specifically found in [basic.stc.dynamic.allocation].