Add Holeberry to community projects

[pedrovieira]

Add Holeberry to community projects

What does this PR aim to accomplish?:

Adds Holeberry, a native macOS menu bar app to monitor and control your Pi-hole instances, to the Community Projects page.

How does this PR accomplish the above?:

Appends an entry to the project list in docs/main/projects.md in the same format as the existing entries (- [Name](url): description) and updates the page's last_updated front matter, following the precedent of previous community-project additions (e.g. #1386).

Link documentation PRs if any are needed to support this PR:

None, this is the documentation change.

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