Add documentation for the PNear ensemble metric

[vmullig]

The PNear ensemble metric computes the propensity to favour a desired state given an ensemble of sampled conformations. As with any ensemble metric, the analysis can be performed for an ensemble of structures on disk, an ensemble generated on the fly in memory (but never written to disk), or even a distributed ensemble sampled on many nodes across a large cluster via MPI (with no single node ever seeing all of the structures). This PR adds documentation for this.

Tasks:

• Update auto-generated docs.
• Add documentation page for PNearEnsembleMetric.
• Add PNearEnsembleMetric to the list of EnsembleMetrics.
• Pull request Update documentation for EnsembleMetrics with MPI #51 must be merged before this one.
• Pull request Add documentation for EnsembleMetrics #50 must be merged before this one.
• This pull request must be merged with the PR (not yet opened) in RosettaCommons/main to add the PNear ensemble metric.