Docs: Explain how to do Debug builds without optimizations on

[sideshowbarker]

This change updates the (advanced) build docs to explain how to do a Debug build with the CXX -O0 option set — which tells the compiler to build with no optimizations at all.

Otherwise, Debug builds use the -Og option — which, when trying to check frame variables in a debugger can result in an error of this form:

error: Couldn't look up symbols: __ZN2AK6Detail10StringBaseD2Ev Hint: The expression tried to call a function that is not present in the target, perhaps because it was optimized out by the compiler.

This is an alternative to https://github.com/LadybirdBrowser/ladybird/pull/2186 and https://github.com/LadybirdBrowser/ladybird/pull/2190.