enable dependabot to auto-generate PRs for when doc-resources submodule is updated

[kbroch-rivosinc]

I don't have access to the settings: https://github.com/riscv/docs-spec-template/settings/security_analysis

I already put the PR in a while back https://github.com/riscv/docs-spec-template/blob/main/.github/dependabot.yml