Major repository cleanup and enhancement

- Reorganize documentation: moved old docs to docs/ directory
- Add comprehensive README.md with build options and usage guide
- Add detailed CONTRIBUTING.md with help content management guide
- Create Makefile for automated building and testing
- Add Dockerfile.scratch for building from Ubuntu 20.04 base
- Enhance all Dockerfiles with PowerShell + PSScriptAnalyzer
- Add modern shells: zsh (with plugins) and fish (with config)
- Add modern CLI tools: fd-find, ripgrep, fzf
- Create comprehensive help system with cheat/TLDR/fish completions
- Add helper scripts for help content management and coverage checking
- Fix Dockerfile.remnux script references
- Support three build variants: upstream (REMnux), scratch (Ubuntu), kali

Build options:
  - make build-upstream: Fast, uses REMnux upstream (recommended)
  - make build-scratch: Full control, builds from Ubuntu 20.04
  - make build-kali: Legacy Kali Linux base

Features:
  - PowerShell with PSScriptAnalyzer module
  - Modern shells (zsh, fish) with custom configurations
  - Enhanced help system (cheat sheets, TLDR pages, fish completions)
  - Help coverage checking and bulk import tools
  - Comprehensive documentation for users and contributors

files/fish_config.fish

@@ -0,0 +1,34 @@
+# REMnux File Analysis Container - Fish Configuration
+
+# Aliases
+alias ls='ls --color=auto'
+alias ll='ls -lah'
+alias grep='grep --color=auto'
+alias fd='fdfind'
+alias rg='rg --color=auto'
+alias analyse='fhelp'
+alias ?='fhelp'
+
+# Fish prompt - simple and clean
+function fish_prompt
+    set_color cyan
+    echo -n 'remnux'
+    set_color normal
+    echo -n '@'
+    set_color blue
+    echo -n (prompt_hostname)
+    set_color normal
+    echo -n ':'
+    set_color yellow
+    echo -n (prompt_pwd)
+    set_color normal
+    echo -n '> '
+end
+
+# Welcome message
+if test -f /opt/README
+    cat /opt/README
+    echo ""
+    echo "🐚 Shell: fish | Type 'fhelp' for help"
+    echo ""
+end