Add links to jsyntrax project

[inponomarev]

This is a proposal to add links to jsyntrax project besides original syntrax.

Some reasons:

• jsyntrax has the same command line arguments and thus it can be used as a replacement for syntrax, particularly in Asciidoctor diagrams.
• Unlike syntrax that requires a number of libraries that are tricky to install outside Linux, jsyntrax is a Java program which can run anywhere Java can run.
• jsyntrax's own documentation is built with Asciidoctor-diagram.
• Here is another example of documentation built with jsyntrax+Asciidoctor-diagram.

[pepijnve]

Thanks @inponomarev, looks like a great addition. For Java-based tools, we can go one step further even and integrate jsyntrax into the Java server (see https://github.com/asciidoctor/asciidoctor-diagram-java). This is a little daemon that is launched once per asciidoctor run. This significantly improves performance for documents with many diagrams since it eliminates the JVM startup cost per diagram. It runs as a separate process (or in-process for JRuby). We would need to make a dedicated jsyntrax type for that I think to know when to activate this logic vs shelling out to the syntrax command.

[inponomarev]

Thanks @pepijnve for quick merge! Yes I will have a look at asciidoctor-diagram-java: it looks like it can cut a lot of time from compiling a documentation with multiple syntrax diagrams.