Update feature list in the documentation

Update feature list in the documentation

In Quick facts section:
 - restructure the Supported platforms section;
 - do not tie KfW to 1.11 release;
 - move references to GSS-API extensions to Feature list table.
In Feature list section:
 - reformat the table;
 - move PRNG and Pre-auth mechanisms into their own tables;
 - clarify GS2 feature description;
 - reference rfc6680 for GSS-API naming extensions.
Lowercase the words in the title of the document.

ticket: 7455