* doc: avoid use of GCC's old syntax (`-Wgnu-designator`)
* doc: highlight shell commands
* doc: point the C++ docs to v8.12
* doc: avoid highlight on output matching bash keywords
- order and group summary
- list by file format type, rather than library
- describe as load/save rather than import/export
- also --vips-config output
I split the VIPS_CFLAGS, VIPS_INCLUDES and VIPS_LIBS var settings into
the various finders for maintainability.
