remove obsolete documentation files

[GrothenDI]

Summary

• remove legacy Markdown files that were excluded from the MkDocs build
• remove orphaned Markdown pages that are outside the configured navigation
• remove the obsolete exclude block from mkdocs.yml
• keep the existing mkdocs-exclude CI dependency available for future use

Validation

• confirmed all 282 remaining Markdown files are present in the navigation
• confirmed there are no missing navigation targets or residual references to deleted pages
• generated the documentation manifest successfully with 282 entries
• ran git diff --check