Add documentation for PDM

[johnh2o2]

@astrobatty has done some great work (awhile ago now) showing PDM some love and care after I abandoned it pretty early on. We should revisit this some more and add appropriate documentation to bring it onto level footing with the rest of the algorithms we have here.

[johnh2o2]

PDM documentation now exists on the v1.0 line (branch v1.0, tip 39d3367): docs/source/pdm.rst, wired into the Sphinx toctree.

It covers the PDM statistic and its sign convention (cuvarbase returns 1 − Θ, so peaks = candidates), all four variants (binned_step, binned_linterp, binless_tophat, binless_gauss) and their _fast kernels, the new (t, y, err) API from #62 with a worked example, the nbins/dphi knobs, and the deprecated legacy format.

Will close this once v1.0 is released and the rendered docs site is rebuilt from it.