Document new Java API for 2.5

[infotexture]

Pull request dita-ot/dita-ot#2604 adds a simple API to invoke DITA-OT from Java. The Developer Reference and related documentation should be extended per @jelovirt's recommendations:

As a developer, my preference would be to ship Javadoc with our HTML documentation and have a tutorial topic on how to use it. The API is small, so a single topic should be enough.

[jelovirt]

The goal of adding the new Java API to DITA-OT is to allow users to embed DITA-OT more easily to Java programs. When using the API, programmers don't need to know or care that DITA-OT uses Ant, they can just use Java. The ant shell script will handle the classpath setup for the user, but when using the API the classpath setup should be done as part of normal classpath configuration.

Example usage:

// Create reusable processor factory with DITA-OT base directory and temporary directory
ProcessorFactory pf = ProcessorFactory.newInstance(ditaDir);
pf.setBaseTempDir(tempDir);

// Create a processor using the factor and configure the processor
Processor p = pf.newProcessor("html5")
    .setInput(mapFile)
    .setOutputDir(outDir)
    .setProperty("nav-toc", "partial");

// Run conversion
p.run();

By default, running DITA-OT via the API will generate a debug log to temporary directory; a custom SLF4J logger can be used to access log.

By default, processor will clean the temporary directory, but this can be disabled for cases where the processor failed.