The GNU C++ library sources have been specially formatted so that with the proper invocation of another tool (Doxygen), a set of indexed reference material can generated from the sources files themselves. The resultant documentation is referred to as the API documentation, and is useful for examining the signatures of public member functions for the library classes, finding out what is in a particular include file, looking at inheritance diagrams, etc.
The API documentation, rendered into HTML, can be viewed locally for the 15.0.0 release, online for each GCC release and for the main development tree (see the date on the first page).
The rendered HTML, as above, is also available for download on the
gcc.gnu.org site in a directory located at
<URL:https://gcc.gnu.org/pub/gcc/libstdc++/doxygen/>.
You will almost certainly need to use one of the
mirror sites to download
the tarball. After unpacking, simply load libstdc++-html-*/index.html
into a browser.
In addition, a rendered set of man pages are available in the same location specified above. Start with C++Intro(3).