templates: add gemini, claude, codex

https://github.com/lima-vm/lima/blob/e827aa401ae765110009ced035f87654c3922275/website/content/en/docs/config/ai/inside/_index.md?plain=1#L6-L9

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Previous discussion:

>> Still not sure whether we should have VM templates such as `template://claude`, as we have never included proprietary software in [our template set](https://lima-vm.io/docs/templates/).
>> Maybe acceptable as `template://nonfree/claude` ?
>>
> I do not think there is any reason not to include example templates for proprietary applications, and I don't think we need "nonfree" in the template name either. I would be fine with something like `template://app/claude` to indicate this is not a distro, but a specific app.
>
> Just like you can install nonfree software with Homebrew, e.g. `brew install docker-desktop` or `brew install multipass`.
>
> Pointing out "nonfree" components seems to be more important to GPL licensed projects.

_Originally posted by @jandubois in [#3712](https://github.com/lima-vm/lima/issues/3712#issuecomment-3053745523)_