Clean up redundant documentation

[krlmlr]

Add stubs, point to tibble's docs.

See tidyverse/tibble#945:

• relink digits vignette
• shorten and relink numbers vignette
• replace ?num, ?char and print.tbl() with internal stubs

[krlmlr]

Need to reexport set_num_opts() and set_char_opts() in tibble.

[krlmlr]

Done.

[github-actions]

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