Docs - enable source highlighting, remove build warnings

Weld documentation doesn't have source highlighting enabled (it now requires attribute) plus there is a number of warnings and info when building the doc.

Source highlighting can use highlight.js which requires no local dependency but cannot be used for PDF files. For those we need some other library, all of which have to be locally installed else a warning is logged during build (but the build continues).

Some of the warnings/infos logged during build:
```
asciidoctor: INFO: possible invalid reference: interceptors
asciidoctor: ERROR: environments.asciidoc: line 3: illegal block content outside of partintro block
```

Weld 4 and 3 might also be affected.