ISC26: update the environments section

[alalazo]

depends on #535

Structural changes:

• Replace the three-section introduction with a tighter motivation using a bullet list
• Renamed "Environment Basics" -> "Working with environments"
  1. "Installing Packages" starts with the add -> concretize -> install workflow, so that is immediately taught to people
  2. "Using installed Packages" is a merge between the previous tcl and mpi use, which felt almost duplicated
  3. "Removing Packagse" now show the easy removal first, and the error later, so it progresses from easy to complex
• Add a new "Environments on disk" section explaining the directory structure, spack.yaml and spack.lock
• Restructure "Sharing environments" around a clear sender/recipient framing

Content changes:

• The content has been updated for Spack v1.2 and Ubuntu 26.04

The output was generated using:

docker pull ghcr.io/alalazo/tutorial-buildcache:isc26@sha256:366369319e0ddee78cf9c727ba8004e7aff4e38068ba4f2ee532c2bceec9168d

[alecbcs]

Looks good to me. Will plan to follow up in another PR for specific wording changes.