Fix similar-pkgs function

[mpadge]

• Fix cosine similarity metrics use in similar pkgs call
• Include vignettes in package text
• Implement chunking of package text, because vignettes often extend inputs to > 8096 tokens so this has to be done anyway.
• Update all reference embeddings locally and attach to GitHub release
• Update README and all pre-generated outputs in vignettes
• Rewrite https://docs.ropensci.org/pkgmatch/articles/E_why-are-the-results-not-what-i-expect.html#why-not-use-chunking-in-pkgmatch

[mpadge]

Vignettes are currently only extracted if .Rmd. That could maybe be extended to include .Rnw, and just dump without any LaTex parsing?