Add GEM to table

[ohno]

Add the results with GEM to the table in index.md.

TwoBody.jl/docs/src/index.md

Lines 101 to 109 in c67ec58

	| Method | ``n=0`` | ``n=1`` |
	|:---|---:|---:|
	| [RR](@ref Rayleigh-Ritz-Method) | -0.499981735104 | -0.124997703473 |
	| [FC](@ref Free-Complement-Method) | -0.499999999978 | -0.123665583532 |
	| [FDM](@ref Finite-Difference-Method) | -0.498756211209 | -0.124921972504 |
	| [QTT+DMRG](@ref Quantics-Tensor-Train) | -0.499702911360 | -0.124981415403 |
	| [VNN](@ref Variational-Neural-Network) | -0.468779111883 | — |
	| [VMC](@ref Variational-Monte-Carlo) | -0.480218518855 | — |
	| Exact | -0.500000000000 | -0.125000000000 |