Update .gitignores for Makefile usage

- allows you to run `git clean -df` and not
  have to remove all the ignored files
- so you now can start with a fresh pkgdown
  after cleaning the `docs` directory out
  (which you cannot do without `git clean -dfx`)
  this would have unintended consequences

.gitignore

@@ -7,5 +7,3 @@ inst/doc/*
 \.DS_Store
 \.Rhistory
 ^r\\.Rproj$
-docs/
-/Meta/