[mdbook] Add patch for .mdbookignore

Mdbook has no way to ignore files when copying the website. There
is an unmerged PR (rust-lang/mdBook#1908)
which adds a .mdbookignore file to do that. This commit cherry
picks this PR as a patch. This is not entirely straightforward
however:
- the PR modifies files which are not found in the release source
  (like the documentation)
- the PR modifies Cargo.toml but the git's version differs from
  the crates.io version (which normalizes Cargo.toml)
- the PR applies on the latest version, but we use version 0.4.43

For this reason, the patch is a trimmed down and manually edit
version of the PR. Furthermore, there is another difficulty:
the patch makes `ignore` a required dependency when previously
mdbook only had it as an optional (and not enabled) dependency,
it also bumps the version. For this reason, this commit manually
adds the required version of `ignore` to the Cargo.toml and uses
rules_rust crate annotations to add a dependency from mdbook to
this version of `ignore`.

Signed-off-by: Amaury Pouly <amaury.pouly@lowrisc.org>

Author: pamaury

MODULE.bazel.lock

@@ -12,3 +12,5 @@ path = "empty_file_to_appease_cargo.rs"
 
 [dependencies]
 mdbook = { version = "0.4.43", default-features = false, features = ["search"] }
+# Required by the patched version of mdbook (mdbookignore feature).
+ignore = "0.4.23"