Make implementations more visible

[chalin]

Followup to #85. From a Slack #tuf comment:

Radoslav Dimitrov: Great work! 🙌 Does it make sense to have a page listing the existing implementations and tools build on top under the official org, i.e. python-tuf, go-tuf, tuf-on-ci, the conformance test suite, etc.? That can help steer people to the right tools.
@chalin: ... something like this : https://docsy--theupdateframework.netlify.app/docs/getting-started/#implementations?
RD: Maybe the suggestion can be updated to have these somewhere more visible

I agree that it can be made more visible. I'd suggest using the third entry (the right-most one) of the "features ribbon" of the site landing page to highlight implementation.