ISC26: update the basics section

[alalazo]

This section has been reorganized, so that concepts are introduced in order rather than interleaved.

Structural changes:

• Intro starts with a few fundamental ideas, that are illustrated in the rest of the tutorial
• "The Spec Syntax" section consolidates all spec coverage into ordered subsections that build up one sigil at a time:
  1. Compiler flags are treated as an aside, instead of a central piece of the tutorial
  2. spack help --spec is the last subsection here
  3. Doesn't show spack find to avoid back and forth
• "Querying Installations" section gathers all spack find usage (including the former "Advanced spack find Usage") in one place.
• "A Realistic Example" section frames the Trilinos install

Content changes:

• The content has been updated for Spack v1.2 and Ubuntu 26.04.
• Doesn't show cflags= but shows +ipo and build_type=Debug instead in the Variants subsection.
• Doesn't show virtual versioning (no mention of mpi@3).
• Removed excessively long output - in particular spack graph for trilinos.

The output was generated using:

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

(see #536 for how that image was built) and editing the Makefile locally to point to that container image.