Vignette links broken in pkgdown

[jwildfire]

[jwildfire]

{pkgdown} and CRAN expect markdown links between vignettes to have .html at the end. So [Intro Vignette](Intro.html) works while [Intro Vignette](Intro) bombs (which was the problem in v2.0). In the GitHub wiki the opposite is true, so we'll just need to update those links between vignettes before releases. Not too bad now that the issue is clear.