Add debug info on getting shell inside runtime + extensions

[hadess]

To be added to either:
https://github.com/flatpak/flatpak-docs/blob/master/docs/debugging.rst
or
https://github.com/flatpak/flatpak-docs/blob/master/docs/extension.rst

We should have a way for developers to be able to get a shell inside a runtime with specific extensions enabled.

Use case would be to be able to see which version of, say, LLVM or Rust, one has available in the extension attached to this version of the runtime.

[...]
runtime: org.gnome.Platform
runtime-version: '48'
sdk: org.gnome.Sdk
sdk-extensions:
  - org.freedesktop.Sdk.Extension.rust-stable
  - org.freedesktop.Sdk.Extension.llvm20
[...]

[hadess]

Oh, it's automatically set:

$ flatpak run --command=sh --devel org.freedesktop.Sdk//24.08
[📦 org.freedesktop.Sdk ~]$ /usr/lib/sdk/rust-stable/bin/cargo --version
cargo 1.89.0 (c24e10642 2025-06-23)

/.flatpak-info has the list of extensions that get loaded.

I'll add some debug to https://github.com/flatpak/flatpak-docs/blob/master/docs/extension.rst too

[hadess]

Thanks for doing this @bbhtt !