feat(cdp): add new button to CDP Overview page

[bijanbwb]

Problem

We already have buttons to create a New source, New transformation, or New destination on the following pages:

• /pipeline/sources
• /pipeline/transformation
• /pipeline/destination

But the Overview page at /pipeline/overview did not have a new button, which could lead to some confusion about how to add a new pipeline.

Credit to @corywatilo for this idea: https://github.com/PostHog/posthog/issues/25566#issuecomment-2412044706

Changes

👀 Pro Tip

If you're viewing the Files Changed tab in GitHub, you can append `?w=1` at the end of the URL and it will show a diff without any indentation and whitespace changes, which makes this PR look a lot bigger than it actually is.

This PR adds a menu button to the Pipeline Overview page, that allows us to create a new source, transformation, or destination.

Does this work well for both Cloud and self-hosted?

It's essentially a routing button that shouldn't impact other features or functionality.

How did you test this code?

It's a UI change only. I clicked the button to verify that each option leads to the correct page.

[github-actions[bot]]

Size Change: 0 B

Total Size: 1.16 MB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `frontend/dist/toolbar.js` | 1.16 MB |

_{compressed-size-action}