Replace underscore with hyphen in article file names and reduce fixture file size

[nanxstats]

Fixes #168
Fixes #121

Vignettes:

• Replace underscores (_) with hyphens (-) in vignettes file names.
• Removed vignettes/articles/figures/readme.PNG as it's not being used anywhere.
• Replace underscores (_) with hyphens (-) for files under vignettes/articles/figures/.
• Update _pkgdown.yml so the articles can build.

Fixtures:

• Re-save data under vignettes/articles/fixtures/. I kept the underscores in the file names following the R Packages book examples.
  • Use the RDS format instead of the RDA format and with xz compression: saveRDS(..., file = "*.rds", compress = "xz").
  • This will make the data more portable (loading without polluting the global environment) and reduce directory size from 66.4MB to 25.2MB.
• Fix the R CMD check note on directory too large by adding a rule to exclude all .html files under inst/check_with_old_version/ to .Rbuildignore.

[nanxstats]

@LittleBeannie ready for your review 💯

Not sure why the GitHub Actions workflows are not triggered, but it builds ok locally.

[LittleBeannie]

Huge scale changes!