Add DigitalOcean fleeting plugin to community plugins

What does this MR do?

Adds the community-maintained DigitalOcean fleeting plugin to the community plugins
table in "Find a fleeting plugin".

The plugin lets the Docker Autoscaler and Instance executors autoscale onto
DigitalOcean droplets. It is published as a signed OCI artifact at
registry.gitlab.com/alexqrid/fleeting-plugin-digitalocean, built against the current
fleeting provider API, with unit tests and an opt-in integration suite running in CI.
It is maintained at
https://gitlab.com/alexqrid/fleeting-plugin-digitalocean.

As required for community entries, the note records the OCI reference, where to report
issues, the support and stability level, and where to find documentation.

Related issues

None.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product availability details are added.
  • Not applicable: this MR adds a single table row and does not change any heading.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.
  • Not applicable: submitted as a community contributor.

/label documentation devopsverify grouprunner-core Category:Runner typemaintenance maintenancerefactor