Add library to mdbook crate

[UebelAndre]

This change adds a library to the mdbook crate so the entrypoint can be consumed without the need for a nightly toolchain to be able to use -Z bindeps (rust-lang/cargo#9096)

closes #2954

[UebelAndre]

Admittedly this change is quite weird but do want to find a conclusion to #2954

For reviewers, lib.rs is a copy of main.rs, no changes were made with the exception of the docstring on main.

[UebelAndre]

re: https://github.com/rust-lang/mdBook/actions/runs/24518893029/job/71670723888?pr=3071

error: running cargo-doc on crate 'mdbook' failed with output:
-----
warning: placeholder v0.0.0 (/home/runner/work/mdBook/mdBook/target/semver-checks/registry-mdbook-0_5_2-default-321f64277e8a6c7d) ignoring invalid dependency `mdbook` which is missing a lib target
error: no library targets found in package `mdbook`

-----

error: failed to build rustdoc for crate mdbook v0.5.2
note: this is usually due to a compilation error in the crate,
      and is unlikely to be a bug in cargo-semver-checks
note: the following command can be used to reproduce the error:
      cargo new --lib example &&
          cd example &&
          echo '[workspace]' >> Cargo.toml &&
          cargo add mdbook@=0.5.2 --features default,search,serve,watch &&
          cargo check &&
          cargo doc

This seems to build locally but I wonder if there's confusion in the lack of a library in the previous release

mbook % cargo new --lib example &&
          cd example &&
          echo '[workspace]' >> Cargo.toml &&
          cargo add mdbook@=0.5.2 --features default,search,serve,watch &&
          cargo check &&
          cargo doc

    Creating library `example` package
note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
    Updating crates.io index
      Adding mdbook =0.5.2 to dependencies
             Features:
             + search
             + serve
             + watch
    Updating crates.io index
     Locking 233 packages to latest Rust 1.95.0 compatible versions
      Adding generic-array v0.14.7 (available: v0.14.9)
      Adding matchit v0.8.4 (available: v0.8.6)
  Downloaded tokio-tungstenite v0.29.0
  Downloaded mdbook-summary v0.5.2
  Downloaded mdbook-preprocessor v0.5.2
  Downloaded mdbook-markdown v0.5.2
  Downloaded mdbook-driver v0.5.2
  Downloaded mdbook-renderer v0.5.2
  Downloaded markup5ever v0.36.1
  Downloaded ego-tree v0.10.0
  Downloaded toml_parser v1.1.2+spec-1.1.0
  Downloaded mdbook-core v0.5.2
  Downloaded html5ever v0.36.1
  Downloaded tungstenite v0.29.0
  Downloaded rand v0.9.4
  Downloaded pulldown-cmark v0.13.3
  Downloaded mdbook v0.5.2
  Downloaded hyper v1.9.0
  Downloaded axum v0.8.9
  Downloaded mdbook-html v0.5.2
  Downloaded 18 crates (1.7MiB) in 0.66s
warning: example v0.1.0 (/Users/user/Code/mbook/example) ignoring invalid dependency `mdbook` which is missing a lib target
    Checking example v0.1.0 (/Users/user/Code/mbook/example)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.91s
warning: example v0.1.0 (/Users/user/Code/mbook/example) ignoring invalid dependency `mdbook` which is missing a lib target
 Documenting example v0.1.0 (/Users/user/Code/mbook/example)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.68s
   Generated /Users/user/Code/mbook/example/target/doc/example/index.html

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.