Description of problem: 

elements in quick start markdown are not present on the first render. so using the QuerySelectorAll with useMemo returns an empty list on first render but since we are using useMemo and dependencies array doesn't detect any change with `document` on the subsequent render even though elements are present in dom.

https://github.com/openshift/console/pull/8527#discussion_r610869592

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

1. The issue can be reproduced in the MarkdownCopyClipboard.tsx or MarkdownExecuteSnippet.tsx by wrapping the document.querySelectorAll call with useMemo

Actual results: 

useMemo returns an empty list

Expected results: 

elements should be present and useMemo should return all the element matching the query selector

Reproducibility (Always/Intermittent/Only Once): always

Build Details:

Additional info: