many improvements to docs based on sphinx

many improvements to docs based on sphinx

- improved code-blocks
- improved json formatting: more compact, replaced // to # for comments
  to make them highlighted
- improved tables proportions
- fixed kea version: |release| should be used instead of KEAVERSION
- changed them to 'Read The Docs' to align with BIND
- cleaned up conf.py
- improved building in Makefile.am, now there are 3 targets (pdf, html, singlehtml)
  and 'all' that depends on those 3
- added kea.css to make html content wider and tables narrow to fit into
  text column