Add high dim example to docs

[AnderGray]

Nearly working new doc page. Shows scalability of Subset algorithms with pf, N_samples, and N_dimensions.

Branch of modified_mcmc

[AnderGray]

The new example likely takes too long to compile in the docs. Should we generate the plots locally, and upload the pngs?

[FriesischScott]

As discussed elsewhere we will introduce a new section in the docs specifically for benchmarks and case studies. These will not be executed when Documenter builds the docs so the images must be created and committed together with the markdown files.

[FriesischScott]

Closed in favour of #175.