pull request for ACTION-350, ACTION-355, ACTION-357

Dear all,


Pull request https://github.com/w3c/sdw/pull/891 packages several changes
that all ease the readability of the specification document, and implement
actions ACTION-350, ACTION-355, ACTION-357:

- changed the way SOSA / SOSA+SSN buttons look and work
- reorganized specification sections to have:
  - one non-normative overview section, with links to the examples that
showcase the terms in that section
  - the normative specification section
- added examples in appendix, and in separate folder, as per ACTION-350 and
ACTION-355
- reorganized terms related to system capabilities in a horizontal module
marked at risk as per ACTION-357
  - and added a "at risk" section in SOTD section
  - and added line in change list
  - and reflected all the changes in the ontology documents and server
specification.

To ease the reviewing process, please find the proposed version of the
document at this URL:

                                http://ci.emse.fr/ssn/


Best,
Maxime Lefrançois