Summarize rootless instructions

[zeehio]

This PR has a balance of 117 additions and 209 deletions.

Six months ago I wrote an article about using rocker in rootless containers.

• Add use rootless page #93

Some settings were complicated due to limitations in the --gidmap syntax exposed by Podman.

As discussed there, I ended up contributing syntax enhancements to Podman.

This PR simplifies the explanations of the rootless page in the website, using the lessons learned while working on Podman. It also focuses on the how it works instead of the why it works, now that the syntax improvements make options more natural.

It preserves instructions for earlier podman versions, although upgrading is encouraged.

[eitsupi]

Thanks!

A few comments.

[zeehio]

Thanks for the quick feedback!

[zeehio]

I'll fix the indentation issues later if you want

[eitsupi]

I'll fix the indentation issues later if you want

It would be helpful!

Also, I would appreciate it if you could clearly indicate "Terminal" in every sh code block like this.

website/use/gui.md

Lines 19 to 21 in 86a5e3a

	```{.sh filename="Terminal"}
	docker run --rm -ti -p 8000:8000 ghcr.io/rocker-org/devcontainer/r-ver:4 R
	```

[eitsupi]

Thanks!

[cboettig]

This looks great to me. My only concern is that it might be better to title this as rootless podman? I understand individuals run in rootless mode in a few different ways (singularity, podman, I believe jupyter-server-proxy on 2i2c and binder setups is rootless, and so forth). It might avoid confusion if this is called rootless-podman.md and the title is something like "running without root privileges using podman" ?

[eitsupi]

My only concern is that it might be better to title this as rootless podman?

Fair point.
There is already a page about singularity.
https://github.com/rocker-org/website/blob/86a5e3a1fbce88741fa69e7f0efbe644425fe67e/use/singularity.md

[zeehio]

Good point! I changed the title :-)

[eitsupi]

Thanks, merging.