sky130: documentation updates

[edwardcwang]

Minor documentation for sky130.

Related PRs / Issues

Type of change:

• [ ] Bug fix
• [ ] New feature
• [X] Other enhancement

Impact:

• [ ] Change to core Hammer
• [ ] Change to a Hammer plugin
• [X] Other

Contributor Checklist:

• [X] Did you set master as the base branch?
• [X] Did you state the type-of-change/impact?
• [X] Did you delete any extraneous prints/debugging code?
• [ ] (If applicable) Did you add documentation for the feature?
• [ ] (If applicable) Did you update the poetry.lock file if you updated the requirements in pyproject.toml?
• [ ] (If applicable) Did you add a unit test demonstrating the PR?
• [ ] (If applicable) Did you run this through the e2e integration tests?
• [ ] (If applicable) Did you update the submodules in e2e/ if this feature depends on updated plugins?