Add a version linking table to docs #444

Add a table in the docs showing:

• python-graphblas version
• suitesparse graphblas version
• version of other backends (once available)
• graphblas spec

The tight pinning of python-graphblas to the implementation and spec will help anyone who needs a specific feature of a backend to figure out the minimum version of python-graphblas to access that feature.

And once the spec changes, this table will also show when new features are ready to use in python-graphblas.