Consolidate the two pages about Maven classloading

### Improvement proposal

We have both
- https://maven.apache.org/guides/mini/guide-maven-classloading.html (https://github.com/apache/maven-site/blob/master/content/markdown/guides/mini/guide-maven-classloading.md) and
- https://svn.apache.org/repos/infra/websites/production/maven/content/reference/maven-classloading.html (https://github.com/apache/maven-site/blob/master/content/markdown/reference/maven-classloading.md)

The latter cannot be accessed via https://maven.apache.org/ due to this redirect rule: https://github.com/apache/maven-site/blob/d0537a942071175ba1300a1006c6630b80031716/content/filtered-resources/.htaccess#L31

The latter should therefore be removed and relevant parts merged into the former.

Read content/markdown/guides/mini/guide-maven-classloading.md and content/markdown/reference/maven-classloading.md, comparing their overlapping classloading guidance and the redirect rule in content/filtered-resources/.htaccess. Done means the relevant reference content is merged into the mini guide and the duplicate reference page is removed.

Written by the indexing model from the issue text.