test/premake: moved threading-support config to togo-deps-link.
doc: removed explicit paths to @file.¹

1: Oh, Doxygen. Was unable to make it accept more-specific
relative paths for headers like types.hpp and gfx/types.hpp,
but @file is still needed to make the first-block commands
apply to the file itself (or at least grouping).
doc/pages: changed @addtogroup to @ingroup.¹

1: This ensures group structure is built. @addtogroup wasn't
*defining* the group, apparently, resulting in no structure
for groups that had no explicit body for the group (`@{` & `@}`).
The body could've been specified in .dox files, but that would've
been more annoying than using @ingroup, which only applies to the
following entities in the current file (or something along those
lines). Additionally, this allows us to forgo duplicating the
group structure in headers, since the structure is now explicit
in .dox files. Yay!
doc: removed group hierarchy duplication from headers.
doc: grouped .dox files in dirs by module.

doc: ensured IO groups have 'io' as prefix, not suffix.
doc/pages: removed old unused gfx_context.doc.