Documentation improvements

[olivroy]

This is a couple of documentation improvements. Addresses #681

• Merges some add / remove documentation together in a single .Rd file
• get/set together
• Removes duplicated entries in the pkgdown index
• Removes redundant @seealso since there is already crossreferences with the @family tag
• Rename R/openxlsx2.R -> R/openxlsx2-package.R
• Remove some mentions to write_data(), write_datatable() in favour of wb_add_data(), wb_add_data_table()
• workbook_grouping.Rd wb_grouping.Rd
• Rename helper functions so that test file and R file align
• Move get_cell_refs tests in test-converters.R as get_cell_refs lives in converters.R

This may not be as focused or thorough as I wished it to be, but these are leftovers that I think can be useful.

Edit: maybe wb_set_col_widths() could be wb_add_col_widths()?

[JanMarvin]

Edit: maybe wb_set_col_widths() could be wb_add_col_widths()?

This might be a good idea indeed.

[Edit:] but then again it should be similar to wb_set_row_heights(). Therefore better just leave it the way it is.

[JanMarvin]

Thanks for the pull requests! Your really helping in a long time overlooked area of the package