Fix missing LICENSE files in published crates

[musicinmybrain]

This adds symbolic links to the top-level license file, ensuring that each crate published to crates.io has a copy of LICENSE.

Before this PR:

$ git clone https://github.com/rust-lang/mdBook.git
$ cd mdBook
$ cd crates/mdbook-core
$ cargo publish --dry-run
$ ls ../../target/package/mdbook-core-0.5.3/
Cargo.lock  Cargo.toml  Cargo.toml.orig  README.md  src

After this PR:

$ cargo publish --dry-run
$ ls ../../target/package/mdbook-core-0.5.3/
Cargo.lock  Cargo.toml  Cargo.toml.orig  LICENSE  README.md  src
$ stat ../../target/package/mdbook-core-0.5.3/LICENSE 
  File: ../../target/package/mdbook-core-0.5.3/LICENSE
  Size: 15921           Blocks: 32         IO Block: 4096   regular file
Device: 0,48    Inode: 20816505    Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/     ben)   Gid: ( 1000/     ben)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2026-06-13 07:13:58.068646413 +0100
Modify: 2026-06-13 07:13:58.067963261 +0100
Change: 2026-06-13 07:13:58.067963261 +0100
 Birth: 2026-06-13 07:13:58.067916421 +0100

[ehuss]

Thanks!

View changes since this review