Add --backend argument for mdbook build command to build the specific backend

[hwhsu1231]

Problem

According to the documentation in mdBook:

If there is more than one backend, then each backend is placed in a separate directory underneath book. For example, the above would have directories book/html and book/wordcount.

Taking the rustc-dev-guide as an example. Since it specifies both html and linkcheck backends in book.toml, running the mdbook build command generates a directory structure like this:

book/
    html/
    linkcheck/

However, I don't want this behavior. I want to generate only the HTML output directly in the book/ directory (without subdirectories like book/html/), even when multiple backends are specified in book.toml.

Proposed Solution

Provide --backend <backend> argument for mdbook build command:

• The default value of --backend is all, which will use all backends specified in book.toml.
• If a specific backend is provided (e.g., --backend html), only that backend will generate output.

For example:

mdbook build --backend html --dest-dir ./book

The above command will generate .html documents directly to ./book directory even if there are multiple backends specified in book.toml. The directory structure should be like this:

book/
    zzz.html
    yyy.html
    xxx.html

Notes

Let's take a look at Sphinx, which provides two arguments for specifying builders (-M and -b):

• -M <buildername>: Outputs files in <outputdir>/<buildername> directory.
• -b <buildername>: Outputs files directly in <outputdir> directory.

What I want is equivalent to the -b behavior—generating files directly in <outputdir> without subdirectories.