data_table: assorted improvements and fixes

[leofontenelle]

Hello! I've been working on the translation of the data_table cheatsheet, and noticed what seems to be some room for improvements or fixes. Specifically:

1. The homepage link points to an obsolete wiki. Please link to https://r-datatable.com.
2. The vignette link points to the functions reference. Perhaps link to the intro vignette?
3. The text boxes with subtitles are set up so that, when you start changing the title (eg for translation) it increases or decreases in size around its center. Ideally it should do so keeping its left margin were it is. In LibreOffice Impress, backtranslating from my native language, this option can be found in menu Format / Text box and shapes / Text attributes... And then, in the Text tab, select the left margin (or perhaps upper left corner) as the text anchor.
4. The text boxes with subtitles should be formatted with text in all upper case, so that you type with title case or sentence case but it is displayed in all upper case. This should improve accessibility (screen readers), spell checking and make it easier for anyone copying-and-pasting said subtitles.
5. The table describing the arguments to melt() has a mismatch between apparent and actual rows. Please make sure each argument/description is in its own row.
6. [edit] Also, there are two data.table cheatsheets ("datatable" and "data_table"), perhaps make sure the new version is the only one.

By the way, data.table 1.16 is due soon