docs: publish structured popgenVCF wiki
Document the new DAPC PC-count cross-validation figure (v1.0.5) Adds explanation and an example image (12b_DAPC_xval_K3.png) for the new 12b_DAPC_xval_K{k} diagnostic figure added in popgenVCF v1.0.5, which visualizes the adegenet::xvalDapc() cross-validation curve already used by default to pick each K's retained PC count. Regenerated from a fresh real reference run against v1.0.5; the other Manhattan-family example figures came back byte-identical (the quickstart dataset is single-chromosome, so the v1.0.4/v1.0.5 label-rendering fixes never trigger for it) except 15_DAPC_loadings_manhattan_K8.png, which differs only by ordinary run-to-run DAPC fit stochasticity.