Move secretgen-controller out of experimental on the website

[aaronshurley]

Describe the problem/challenge you have
On carvel.dev, secretgen-controller shows under the Experimental Tools section though it has graduated out of experimental some time ago (see this issue). We want to give users confidence in this tool so we should update its position on the website.

Describe the solution you'd like
Acceptance Criteria:

• secretgen-controller is promoted out of the Experimental Tools section
• secretgen-controller is listed under the Projects dropdown (it can just link to the GitHub repo for now, no micro-site necessary)

Anything else you would like to add:
We previously halted this effort when we were going to redesign the homepage but that has been paused indefinitely. From a timing perspective, we'd like to get this done before going under review for the CNCF Sandbox (estimated mid-September).