doc: put example of enableExternalDtdLoad to xdoc

[romani]

taken from #10356 (comment)

error message modified a bit to match limit of 100 symbols, "Name" word is removed, I think it will be not noticable.

[rnveach]

#10356 (comment)
I feel this documentation does not completely detail why this is hidden behind a property and what are the security issues with enabling it without any thought.

[romani]

Github, generate website

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/9c57f89_2022143905/index.html

[rnveach]

[ERROR] Failures: 
[ERROR]   XdocsPagesTest.testAllSubSections:347 config_system_properties.xml sub-section Examples for section Enable External DTD load must match
expected: Enable_External_DTD_load_Examples
but was : External_DTD_Examples
[INFO] 
[ERROR] Tests run: 3824, Failures: 1, Errors: 0, Skipped: 2

[romani]

build problem is fixed.

[romani]

Github, generate website

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0e23163_2022195814/index.html

[romani]

Github, generate web site

[romani]

$ cat checkstyle-test.xml

I now feel that is better to remove this and similar cats line from doc. do you agree ?

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/2fe6034_2022203300/index.html

[rnveach]

I am ok to remove cat lines if it is made explicitly clear what each file is.

[romani]

Github, generate web site

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/8093ecd_2022211354/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/8093ecd_2022211354/config_system_properties.html