Fix the error of unable to build the pkgdown site

[vedhav]

The pkgdown::build_site() throws error:

-- Building function reference -------------------------------------------------
Error:
! in callr subprocess.
Caused by error in `check_missing_topics(rows, pkg)`:
! All topics must be included in reference index
✖ Missing topics: cli_theme_tests, expect_attr_width, minimal_metadata, minimal_table
ℹ Either add to _pkgdown.yml or use @keywords internal
ℹ See `$stdout` and `$stderr` for standard output and error.
Type .Last.error to see the more details.