Add IPC Shared Mem recipe to the ReadTheDocs

[kab163]

Currently the IPC Shared Memory cookbook page can only be found in the repo as a .cpp file, but it should also be added to the ReadTheDocs as a .rst file

[kab163]

Other notes about IPC shared memory for documentation:

• A note about file descriptors used for shared memory. Rao had an issue with this - see email.
• A note about how the name in rm.makeResource must contain a "SHARED_" prefix for shared allocators
• To find how much memory is available for a shared memory allocator, just call getActualSize() on the allocator (see question in Umpire general teams chat)