docs: lead installation guide with Docker/Apptainer over from-source

[sevmag]

Closes #889.

Reorganises docs/source/installation/install.rst so the recommended path — the pre-built Docker/Apptainer images — comes first, with from-source install framed as the option for writable/editable installs or systems without a container runtime. No technical content changes: just ordering and framing.

What changed

• Quick Start now opens with a short pointer to the Docker/Apptainer images as the recommended path, with a cross-reference to Installing From Source_ for writable/editable installs.
• Docker & Apptainer Images moved to the top, framed as the default. Added a note that Apptainer is the standard path on HPC clusters.
• Installing From Source follows, with a lead-in explaining when to use it (development/editable installs, or no container runtime).
• The IceTray subsection now points up to the IceTray+GraphNeT image as the preferred route and explains why (bundles a known-good IceTray + GraphNeT combination, avoids version-pinning issues); the CVMFS recipe is retained as a fully-supported alternative.
• Minor whitespace cleanup (trailing spaces, missing final newline).

Motivation

HPC users often can't install from source on a bare cluster — e.g. RHEL/Rocky 8 nodes ship glibc 2.28, while pyg_lib's libpyg.so in the PyPI wheels requires GLIBC_2.29, so pip install -e .[torch-XX] outside CVMFS/conda-forge tends to fail at import time. The pre-built images sidestep this entirely and are autobuilt per release for both CPU and GPU. Leading with them better matches what's actively maintained.

🤖 Generated with Claude Code

[sevmag]

Feel free to add suggestions that change the wording or text now that we are already reworking this section @Aske-Rosted @christianlocatelli

[christianlocatelli]

Maybe here you could emphasize that this option is the recommended one for contributors.
"Installing From Source (for development, or if Docker/Apptainer isn't available)"

[christianlocatelli]

I would adapt this section a bit. Instead of "easiest way to get started", I would suggest to instead use something like: "The most reliable way for GraphNeT Users to get a working environment...".

I would also already mention in that section, that the Docker option is not available for KM3NeT so far. Otherwise KM3NeT Users reading it from top to bottom could be a bit mislead. Could we add a one-line qualifier in the intro, e.g. "Docker images are currently available for IceTray-based experiments (IceCube & P-ONE); KM3NeT users should install from source for now"?

[christianlocatelli]

I don't know how familiar people in the community are with Docker/Apptainer. I personally never used containers before and would therefore have to look some things up before installing it.
In the Source installation section there is a link leading people to the Anaconda installation, so I thought it could be useful to provide at least a some links to the get started pages of Docker and Apptainer somewhere in this section. This is just a minor change but it leads ppl who are unfamiliar immediately to the most relevant pages.

Smth like:
(New to containers? See the get-started guides for Docker and Apptainer.)

[christianlocatelli]

Hi Severin, I left some comments on your PR. Let me know what you think :)