Add warning box about system dependencies

[rgaiacs]

The documentation of requirements.txt and install.R does not mention the need to use apt.txt. This might be only relevant to 5% of repo2docker users but it will make our documentation friendly to beginners.

[manics]

There's a past discussion (unfortunately I can't find it, it was several years ago) where we discussed deprecating apt.txt because it's unavoidably tied to the base image. Upgrading the Ubuntu base image means apt packages may have their naems changed, having breaking upgrades, or be renamed. Using conda packages with a pinned Python or R version should be more robust.

[rgaiacs]

Using conda packages with a pinned Python or R version should be more robust.

The way that I see is that we "soft" deprecate the support of requirements.txt, install.R, pyproject.yaml, apt.txt in favor of conda-forge and require users to contribute their packages to conda-forge. Or we properly disclose edge cases when a configuration file requires another configuration file.