documentation: search data format version 2.

documentation: search data format version 2.

Instead of having a hardcoded mapping from flags to type names and
associated classes on the JavaScript side, which was very C++-specific,
the new format now moves this mapping directly into the search data.

It's a new section at the end, for each type index containing CSS class
ID (which is still hardcoded, but that's not language specific anyway)
and an UTF-8 name that's displayed in the label. For C++ this makes the
search data file size 107 bytes larger, which isn't significant by any
means.