Better tagging directory structure

[mathbunnyru]

Describe your changes

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

There were quite a lot of files in tagging/ directory.

So, I:

1. Moved wiki related files to the separate wiki/ directory
2. Reorganised the directory structure of tagging as follows:

tagging/
├── __init__.py                      # Package marker
├── apps/                            # Command-line applications
│   ├── __init__.py
│   ├── apply_tags.py               # Apply generated tags to images
│   ├── common_arguments.py         # Shared CLI argument parsing
│   ├── merge_tags.py              # Create multi-arch images
│   └── write_tags_file.py         # Generate image tags
├── taggers/                       # Classes for generating image tags
│   ├── __init__.py
│   ├── date.py                   # Build date tagger 
│   ├── program_versions.py       # Version taggers for Python, R, Julia etc.
│   ├── sha.py                    # Git commit SHA tagger
│   ├── tagger_interface.py       # Base interface for taggers
│   └── ubuntu_version.py         # Ubuntu version tagger
├── manifests/                     # Classes for generating build manifests
│   ├── __init__.py
│   ├── apt_packages.py           # APT packages manifest
│   ├── conda_environment.py      # Conda packages manifest
│   ├── header.py                 # Build manifest header
│   ├── julia_packages.py         # Julia packages manifest
│   ├── manifest_interface.py     # Base interface for manifests
│   ├── r_packages.py            # R packages manifest
│   └── spark_info.py           # Spark version manifest
├── hierarchy/                    # Image relationship definitions  
│   ├── __init__.py
│   ├── get_taggers_and_manifests.py   # Get taggers/manifests for image
│   └── images_hierarchy.py            # Define image dependencies
└── utils/                      # Shared utilities
    ├── __init__.py
    ├── docker_runner.py       # Docker container management
    ├── get_platform.py       # Platform detection
    ├── get_prefix.py        # Path prefix helpers
    └── git_helper.py        # Git repository utilities

3. Note that each Tagger and Manifest got its own file instead of having one long file with all of them. I kept versions.py together though, because these are similar.
4. Updated tagging/README.md and made it more clear
5. Moved this README to docs/maintaining/tagging.md
6. Replaced all pieces of code in this file with literalinclude

I also made sure that git think files are moved (and not deleted and added), so it should mess less with the history