[docs] fix msbuild template

[SpaceIm]

There is a big confusion in current assumption of the interaction between MSBuildToolchain/MSBuildDeps generators and MSBuild helper: actually there is no automagic injection of these props files in MSBuild currently.
I've proposed conan-io/conan#12817 which should allow smooth usage like other families of generators + build helper (CMakeToolchain/CMakeDeps + CMake, AutotoolsToolchain/AutotoolsDeps + Autotools, MesonToolchain/MesonDeps + Meson etc), but meanwhile this msbuild template is broken. So here is a workaround.

Moreover:

• Most of the time, configuration attribute of MSBuildToolchain & MSBuildDeps should be set (usually to the same value than build_type attribute of MSBuild helper, but it can be more tricky).
• platform attribute of MSBuild should not be overridden usually (and if it's overridden, the one of MSBuildDeps should also be overridden accordingly I guess). The default one is more robust than the one proposed previously in this template.
• No need to call VCVars.generate(), it's already called under the hood by MSBuildToolchain.generate() (and actually it's more robust because it handles IntelCC).

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.