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