Add official support for `spacingFactor` in the `preset` layout

[maxkfranz]

Description of new feature

What should the new feature do? For visual features, include an image/mockup of the expected output.

The preset layout already supports the spacingFactor option, like many other discrete layouts do. However, it's not officially supported or documented.

Document the feature and list it as officially supported.

Motivation for new feature

• The feature already works. Just document it.
• Expanding/Collapsing of clusters enrichment-map-webapp#184

Describe your use case for this new feature.

For reviewers

Reviewers should ensure that the following tasks are carried out for incorporated issues:

• Ensure that the reporter has adequately described their idea. If not, elicit more information about the use case. You should iteratively build a spec together.
• Ensure that the issue is a good fit for the core library. Some things are best done in extensions (e.g. UI-related features that aren't style-related). Some things are best done by app authors themselves -- instead of in Cytoscape libraries.
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the unstable branch via pull request. The corresponding pull request is cross-referenced.

[stale]

This issue has been automatically marked as stale, because it has not had activity within the past 14 days. It will be closed if no further activity occurs within the next 7 days. If a feature request is important to you, please consider making a pull request. Thank you for your contributions.