Update std.container.* docs to reflect submodules

After `std.container` was split into submodules, the docs were not properly updated to reflect the new state of things. This PR fixes that.

Depends on: https://github.com/D-Programming-Language/phobos/pull/2771 (otherwise the new docs won't actually get built by the makefile). Also https://github.com/D-Programming-Language/dlang.org/pull/717 to update the navbar on dlang.org.