All xdoc examples are being moved to real java tests in #13100

The purpose of this issue is to gather requirements on how this parser should work and then implement it after an agreement is achieved.

• There is a proposal at BDD configuration comments should be in XML #12155 (comment) to use real XML configs.
• We should remove "package" in HTML version of this snippet.

Edit: It was decided to use Doxia Macros for that. Doxia already comes with a built in SnippetMacro that works in a similar way to what we want - use delimiter comments inside the target file. We will just build a similar one that fits our needs. Doxia documentation already explains how to make our own macro - https://maven.apache.org/doxia/developers/macros.html. Source code of snippet macro - https://github.com/apache/maven-doxia/blob/master/doxia-core/src/main/java/org/apache/maven/doxia/macro/snippet/SnippetMacro.java

Since we want to keep the xdoc files in the repository, a custom parser needs to be created to just execute the macros and keep the rest of the xdoc the same. This parser will work very similarly to the default xdoc one - https://github.com/apache/maven-doxia/blob/master/doxia-modules/doxia-module-xdoc/src/main/java/org/apache/maven/doxia/module/xdoc/XdocParser.java
Doxia documentation on how to build a custom macro - https://maven.apache.org/doxia/developers/modules.html.