Autogenerate contents table if user doesn't supply one

[matt-dray]

new_a11ytable() captures all the information required to make the contents page without the need for the user to create it themselves. It's just tab_title and sheet_title from the a11ytables-class object in a table format.

Would users want to add anything more to the contents table? Might be beneficial to be restrictive and stop this (better consistency, fewer inputs for end user, less chance for contents/tabs mismatch errors as per #34). They can always add things after the a11ytable has been created.

So for this demo dataset:

> mtcars_df
# # A tibble: 4 × 6
#   tab_title sheet_type sheet_title                                          source             table_name        table        
#   <chr>     <chr>      <chr>                                                <chr>              <chr>             <list>       
# 1 Cover     cover      The mtcars demo datset: 'Motor Trend Car Road Tests' NA                 cover_sheet       <df [2 × 2]> 
# 2 Contents  contents   Table of contents                                    NA                 table_of_contents <df [2 × 2]> 
# 3 Notes     notes      Notes                                                NA                 notes_table       <df [6 × 2]> 
# 4 Table 1   tables     Motor Trend Car Road Tests                           Motor Trend (1974) car_scores        <df [32 × 6]>

You can grab the tab and sheet titles and put them in a data.frame:

x <- mtcars_df[mtcars_df$sheet_type %in% c("notes", "tables"),  ]

contents_df <- data.frame(
  `Sheet name` = x$tab_title,
  `Sheet title` = x$sheet_title
)

Or whatever.

[matt-dray]

I think it might be conceptually weird for the user not to provide the contents table, since they'll be providing a table for all the other sheets. I think this provides maximum flexibility for the user as well, who might want to add more colums into the contents table for whatever reason and they should be able to do it at the step where they add the table, rather than having to create the a11ytable and then amend it afterwards.

[matt-dray]

Better: generate the notes sheet on behalf of the user if they do not provide the information to create one. This would maintain backwards compatibility too.