Latest sitegen and functional docs improvements

[romain-grecourt]

Doc Updates

Latest sitegen-maven-plugin

• Better nav composition (supports "flattening")
• Better error reporting !! (aggregated errors, pseudo stack trace)
• Nav processing happens first (fail-fast, opposed to post rendering)
• Strict error checking !! (xrefs, warnings)
• Tool-chain alignment for better productivity
  • allow use of other standard rendering: IntelliJ, asciidoctor cli, GitHub etc.

Functional doc changes

• Updated xref syntax (from <<file.adoc,tex>> to xref:file.adoc[text])
• Fixed all broken references
• Enumerate pages in nav to declare the ordering (instead of using natural sorting and prefixes)
• Removed XX_ prefixes !!!
• Sorted rendering (more predictable)
• Page naming convention (dash separated is the now the norm)
• Conventions for includes, everything is derived from {rootdir}
  • Every pages MUST define :rootdir: AND include one of includes/attributes.adoc, includes/se.adoc, includes/mp.adoc
  • Common attributes (includes/attributes.adoc) ; contains versions, versioned URLs (specs, javadocs etc.)
• Flavor attributes (includes/se.adoc, includes/mp.adoc) ; includes includes/attributes.adoc and sets flavor specific attributes

[danielkec]

Streams, Messaging and LRA look ok