docs: reorganize installation guide and add HPC documentation

[yohaimagen]

Documentation: Reorganize Installation and Add HPC Guides

Description

This PR updates and reorganizes the tandem documentation to improve clarity and include detailed instructions for several High-Performance Computing (HPC) systems.

Key Changes:

• Reorganized Installation Guide: The installation.rst page has been streamlined to focus on manual installation.
• Dedicated Spack Page: Created spack.rst to house all Spack-related installation and configuration details, including environment module generation.
• New HPC Section: Added a dedicated hpc/ directory with comprehensive guides for:
  • SuperMUC-NG: Updated with environment setup, Spack repository configuration, branch-specific builds, and SLURM job submission examples.
  • LMU-HPC: for hisenbug and kernelpanic
  • Frontera: Added a guide for installation and compilation on the Frontera system.
• Indexing Updates: Updated the main Getting Started index to reflect the new structure.

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Branch Dependencies & Merge Order

This PR is branched from PR #96, as I added the docs over there for the release Docker images and changed the docs index accordingly. To avoid conflicts, this branch should be merged into #96 first, then merged downstream.

Merge Order Summary:
downstream <- PR #96 <- This PR

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[JeenaYun]

Hi @yohaimagen, I finished reviewing this PR. Mostly looks great, just a few stale statements need to be removed.

[JeenaYun]

Thanks for addressing all the suggestions. The PR now looks good to me.

[JeenaYun]

@yohaimagen I pushed two commits. The first adds the tandem logo to docs/images and applies it to 1) the README.md file and 2) to the sidebar of the readthedocs page. The other is fixing minor typos in 1) BP6 example mesh generation script (was in meters while needed to use kilometers) and 2) docs/first-model/run-model.rst to correct the usage of mode="QDGreen". Please let me know if any of them need to be fixed.

[yohaimagen]

@piyushkarki

This PR is ready to merge. All review comments from @JeenaYun have been addressed, and the latest additions (Tandem logo in README/sidebar and the BP6/run-model typo fixes) look good.

Regarding the CI failures — they are not caused by any change in this PR. The failures in tests 22–27 (pre-clean-test-outputs, generate-test-outputs, etc.) are due to a missing script test/test_data/scripts/cleanup_test_outputs.sh. I am not sure how it popped up, but here we changed only docs-related files and the README

[yohaimagen]

See Branch Dependencies & Merge Order in the first message of this PR.

[Thomas-Ulrich]

Just as a quick note, there was #70
documenting installation for GPU, Leonardo and LUMI.
(but I guess this can be merged later one).