fix: keep doc/ and Meta/ out of the build

Removing ^vignettes$ was right; removing ^doc$ and ^Meta$ with it was
not. Those are devtools::build_vignettes() artefacts, not sources -- R CMD
build regenerates inst/doc/ from vignettes/ by itself, and shipping the
local copies earned a 'non-standard file/directory found at top level'
NOTE.

R CMD check --as-cran is now 0 errors, 0 warnings, 0 notes.

.Rbuildignore

@@ -3,6 +3,8 @@
 ^\.Rproj\.user$
 ^_pkgdown\.yml$
 ^docs$
+^Meta$
+^doc$
 ^pkgdown$
 ^\.github$
 ^LICENSE\.md$