deprecate the book.text-direction field from the configuration

[szabgab]

Problem

It is a bit confusing to have the text-direction field as it is not needed if the language field is set to one of the RTL languages.

It seems that the text-direction is now derived from the language field of the book section of the configuration.

According to the list of public mdbooks there is only one book using the separate text-direction field. I asked them to verify that the language-field is enough for them. persian-rust/book#6

Of course there are probably many other RTL books out there, some public some private.

Proposed Solution

I'd suggest to add a warning to mdbook if it encounters the use of the text-direction field. In the report mention that the language field should be enough and ask them to report here if for some reason they need the text-direction field.

Then, if there is no objection, a few months later remove this field from mdbook.

Notes

Current version mdbook v0.4.48