Document in more places that `output-dir` will be deleted

The output directory is (usually) completely deleted (see eg #13623). In the related issue #13892, @mcanouil said

See quarto render --help .

However, quarto render --help does not actually say anything about this rather important point. Currently, this is what it says:

  --output-dir                        - Write output to DIR (path is input/project relative)

and I propose changing this to, perhaps:

  --output-dir                        - Write output to DIR, which is first deleted if it exists (path is input/project relative)

This misses some subtlety, of course.

I'm currently trying to find where in the docs it actually says that the directory will be deleted. I'm sure it says it somewhere, but it's a bit hard to find. I would suggest adding this VERY IMPORTANT caveat to all the places it discusses output-dir.

For instance, on project options:

Same thing on project options for websites:

Similarly, this fact is not mentioned on the Output Path section for books.

Tangentially, this is not mentioned either in at least this place in TPG.

Start with quarto render --help and the linked project options, website project options, book output path, and TPG project options pages to locate every output-dir description. Update the relevant documentation and help text so the deletion caveat is stated wherever output-dir is discussed, then verify the rendered pages and help output.