ci: harden docs workflow_dispatch version handling

## Summary
The manual docs deploy path in `.github/workflows/docs.yml` interpolates `github.event.inputs.version` directly into a `run:` script. The release path in the same file already binds the version via `env:` / `$MIKE_VERSION`.

## Proposed fix
Bind `workflow_dispatch` version under `env:` before `mike deploy` (quoted shell var), matching the release step pattern.

## Note
CI hygiene / defense-in-depth — not filed as a vulnerability advisory.