#1235 AsciiDoctor-based documentation: prevent duplicate CSS file on website + consistently put CSS files in 'assets' folder

• Best to review per commit.
• Changes are not end-user visible:
  • Prevents double CSS file in website root and assets subfolder.
  • Now website and Eclipse help both have CSS file in assets folder.
  • Now all CSS files are consistently in the assets folder.
• Detailed changes:
  • Moved AsciiDoctor CSS files into assets sub-folder in source projects.
  • Updated AsciiDoctor configuration for moved CSS file.
  • ToolDef website assembly: change assembly id. Was copied from elsewhere, but never updated. Other docsets already used this id, so now it is also consistent.

Closes #1235 (closed)