Update XML_External_Entity_Prevention_Cheat_Sheet.md

[KadirArslan]

I've added quick summary for impacts and security matrix. I think this is important for quick scan to document!

:triangular_flag_on_post: If your PR is related to grammar/typo mistakes, please double-check the file for other mistakes in order to fix all the issues in the current cheat sheet.

Please make sure that for your contribution:

• [x] In case of a new Cheat Sheet, you have used the Cheat Sheet template.
• [X] All the markdown files do not raise any validation policy violation, see the policy.
• [X] All the markdown files follow these format rules.
• [] All your assets are stored in the assets folder.
• [] All the images used are in the PNG format.
• [X] Any references to websites have been formatted as [TEXT](URL)
• [X] You verified/tested the effectiveness of your contribution (e.g., the defensive code proposed is really an effective remediation? Please verify it works!).
• [X] The CI build of your PR pass, see the build status here.

AI Tool Usage Disclosure (required for all PRs)

Please select one of the following options:

• [X] I have NOT used any AI tool to generate the contents of this PR.
• [ ] I have used AI tools to generate the contents of this PR. I have verified the contents and I affirm the results. The LLM used is [llm name and version] and the prompt used is [your prompt here]. [Feel free to add more details if needed]

[szh]

I'm going to let the others who have more subject knowledge review this.

[KadirArslan]

can someone check this one? ☝️

[szh]

@mackowski can you please review this?