Improve IFrames src

[martinRenou]

Description

The iframes sources are defined as https://whatever.readthedocs.io/lite/replite/..., "lite" being the directory in which JupyterLite is built.
This causes an issue when the iframe is embedded in a docs sub-directory, as the iframe link would append the directory to the URL: https://whatever.readthedocs.io/subdir/lite/replite/... resulting in a 404 issue.

I tried fixing this in #32, which was working just fine in some cases. But it seems to have broken the jupyterlite-sphinx docs as reported by @psychemedia in #34

This fix was reverted for now, in #35, but I believe we should find a way to fix the sub-directory issue mentioned above.

We probably need to be smart about the URL of the iframe, trying to find the lite path relatively to the the sub-directory that's calling the directive.

[psychemedia]

There's a general question, I think, of how to introspect or communicate the domain and path that lite environments live on.

I started looking at updating a sphinx/Jupyter Book theme to allow launching the notebook used to generate a sphinx rendered HTML page in retrolite, and for that I had to assume the location of the retro site rather than eg try to pick up on a config setting.