Allow adding footnotes without a location parameter

[rich-iannone]

Discussed in #843

^{Originally posted by JohannesNE October 27, 2021}
It would be useful to add footnotes with tab_footnote() without specifying a location.
This should just add an entry to the footnote text without a superscript number.

My specific use case is to explain a symbol used in the table.

tab_source() can nearly do this, but it is displayed in a separate section