fix(docs): remove multilingual config field

Problem: "deploy docs" always pulls in the `latest` release of `mdbook`,
which now is a v0.5.0 prerelease with breaking changes -- including
removing an (apparently unused) `multilingual` config field in the TOML
that is now an error (another breaking change).

Solution: Delete the line. Add `workflow_dispatch` to the docs workflow
in case follow-up changes are needed; see
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#05-migration-guide

.github/workflows/docs.yml

@@ -3,6 +3,7 @@ on:
   push:
     branches: [master]
     paths: [docs/**]
+  workflow_dispatch:
 
 jobs:
   deploy-docs: