feat: Add `skip_local_inventory` option to not register objects in the local inventory

Closes #296, a follow-up to mkdocstrings/mkdocstrings#671. In this PR, I'm installing `mkdocstrings` from the mkdocstrings/mkdocstrings#774 PR to build upon the pending feature. As soon as it's merged, I'll revert d57c79c5fcfd6ff519b913c57563689d4facf385.