based on discussion at #15799
where update becomes too big to be done at ones, so we need to split update in few phases.

updates:

• remove usage of xdocDirectory property in plugin as it is deprecated; https://maven.apache.org/plugins-archives/maven-site-plugin-3.6/run-mojo.html#xdocDirectory , dependency: bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0 #15799 (comment)
• find way to use custom css and js files
• usage of skin as artifact
• migration to maven-site-plugin 3.21.0
• enable xml plugin in pom.xml
• fix links in sun and Google style, remove them from suppression that we did in first PR
• mobile web site should have tables to be single column.
• non-formatted text should be wrapped , very visible in mobile mode. Attention to <suppress checks="OrderedPropertie.. https://checkstyle.org/checks/misc/orderedproperties.html#Description vs https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/736f882_2025165436/checks/misc/orderedproperties.html#Example1-code
• all table headers column text should be adjusted to left. Not a middle of cell. https://checkstyle.org/checks/misc/orderedproperties.html#Properties
• code blocks should not be wrapped. Acute problem for mobile mode. https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/736f882_2025165436/checks/misc/orderedproperties.html#undefined
• remove text links under banner images in left panel, just image/banner is enough. https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/736f882_2025165436/index.html
• click on name of section use to update link in browser location to have full link with anchor. Kill feature for maintainers to share link to exact place in doc. Now it is generating #undefined https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/736f882_2025165436/checks/annotation/annotationlocation.html#undefined
• no link images as first column, https://checkstyle.org/cmdline.html#Command_line_usage_Command_Line_Options vs https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/736f882_2025165436/cmdline.html
• not clear why some text under section has different indentation from left - https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/077463c_2025133942/checks/annotation/annotationlocation.html#AnnotationLocation , search for "Attention: Elements that cann...."
• in logs there are huge amount of WARNINGS like: Anchor name "Description" used more than once, Anchor name "Properties" used more than once, ....
• Make hamburger stick to the top, not done but not clear, non sticky is good .
• left panel links/text is not wrapped dependency: bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0 #16077 (comment)
• in mobile mode reduce left and right margin, to increase space for text as much as possible. Details dependency: bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0 #16077 (comment)
• header images is bigger than before and make redline is not touching top of page, details.
• tables headers text should aligned to top of cell, details.
• make line spacing for code snippets (line height) to be compact, as in was in old website
• code snippets have too much in top/bottom spacing details