mdbook-i18n-helpers: Initial integration (#11997)

[mdbook-i18n-helpers](https://github.com/google/mdbook-i18n-helpers/) is
a plugin for [mdbook](https://rust-lang.github.io/mdBook/) which enables
localization support.
According to [crates](https://crates.io/crates/mdbook-i18n-helpers), it
was downloaded ~100k times.

The project already defines [several fuzzers within its
repo](https://github.com/google/mdbook-i18n-helpers/tree/main/fuzz).

I am one of the maintainers of the project. The main maintainer
@mgeisler approves of this request as can be seen from
google/mdbook-i18n-helpers#83.

projects/mdbook-i18n-helpers/project.yaml

@@ -0,0 +1,7 @@
+homepage: "https://github.com/google/mdbook-i18n-helpers"
+main_repo: "https://github.com/google/mdbook-i18n-helpers.git"
+language: rust
+primary_contact: "[email protected]"
+auto_ccs:
+  - "[email protected]"
+  - "[email protected]"