Add docs about BEAST output files

[lea-hagen]

The documentation doesn't currently have anything about what output files are created (stats, lnP, PDFs). There's a page describing the columns in the stats file, so maybe add a bulleted list to the fitting section with links to the output file descriptions (which, incidentally, might be good to combine with the grid descriptions).

[lea-hagen]

Update: as of #495, info about stats and 1D/2D PDF files are in the docs. Just need to add lnP info.