rust: kbuild: clean output before running `rustdoc`

`rustdoc` can get confused when generating documentation into a folder
that contains generated files from other `rustdoc` versions.

For instance, running something like:

    rustup default 1.78.0
    make LLVM=1 rustdoc
    rustup default 1.88.0
    make LLVM=1 rustdoc

may generate errors like:

    error: couldn't generate documentation: invalid template: last line expected to start with a comment
      |
      = note: failed to create or modify "./Documentation/output/rust/rustdoc/src-files.js"

Thus just always clean the output folder before generating the
documentation -- we are anyway regenerating it every time the `rustdoc`
target gets called, at least for the time being.

Cc: [email protected] # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Reported-by: Daniel Almeida <[email protected]>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089/topic/x/near/527201113
Reviewed-by: Tamir Duberstein <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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