Mention JupyterLab Desktop on "Install" and "Try" pages

[JasonWeill]

The JupyterLab Desktop app provides a streamlined installer so that users less familiar with CLI tools can use notebooks on a personal computer. The "Try" and "Install" pages mention some of our apps, but not JupyterLab Desktop.

I spoke with @mbektas and he was OK with us promoting Desktop in more places, including on our web site. Let's add it as a first-class project on jupyter.org.

[krassowski]

I mean, yes but it would be so nice if we could get the issue with it being flagged as a security risk on widows resolved first. For the sake of project reputation and promoting good practices I think we should not be telling people "your OS will flag it as a potential virus but don't worry you can ignore it".

jupyterlab/jupyterlab-desktop#202 (comment)

[JasonWeill]

This is a duplicate of #589, but let's leave it open, in the hope that we can get this one done.

Related issue on JupyterLab Desktop repo: jupyterlab/jupyterlab-desktop#275

[JasonWeill]

Still blocked on code signing certificate, as @krassowski mentioned earlier: jupyterlab/jupyterlab-desktop#202

[hxrshxz]

kindly have a look at the PR and tell if any other changes need to me made