Move Explain Vulnerability components to own folder

What does this MR do and why?

Currently the Explain Vulnerability components are in a folder with 20+ other components. This MR moves them to their own sub-folder so that it's easier to find them.

It also renames ExplainThisVulnerability/explain-this-vulnerability to ExplainVulnerability/explain-vulnerability. While the feature is called "Explain This Vulnerability", on the backend and in several frontend components we use ExplainVulnerability/explain_vulnerability, so the rename is to standardize on one name instead of two.

There are no feature changes, this is a simple move/rename refactor.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #420745 (closed)