docs: align custom and quick package installation

[VolkerChristian]

Owner-approved installation structure

All six distributions now use Quick install → Complete sets, then Custom install → Selected packages. Commands begin immediately after the Selected packages heading; catalog guidance follows the checks. The redundant Complete sets / Selected packages list is removed. Preserve #choose-packages as an explicit anchor, all remaining targets and every shell command block.

The .github navigation plan generates the Custom install label through the existing renderer; DistributionPackages owns the six guides and renamed generated SVG. No workflow, package state, signing, installer, feed or key changes.

Verification

• Six-guide assertions for heading order, direct command placement, catalog placement and removal of redundant lists: passed.
• All fenced command blocks byte-identical to origin/main: passed.
• docs-audit: 0 violations for both source repositories.
• page-headers --repo DistributionPackages/site --check: passed.
• git diff --check: passed.
• Production-code growth: zero; documentation/plan/assets only.