generate_config_doc() was writing configure.rst directly into the source
tree, which fails when building from a read-only source directory (e.g.
when the source is on a read-only filesystem or in a packaging
environment).
The Sphinx build in libc/docs/CMakeLists.txt already copies static .rst
files from the source tree into the build tree so that generated docs
don't pollute the source directory. Move configure.rst generation to
follow this same pattern by writing to LIBC_BUILD_DIR/docs/ instead of
LIBC_SOURCE_DIR/docs/.
This also removes configure.rst from the checked-in source tree, since
it was fully generated content that was being regenerated on every CMake
configure anyway.
253616de7e