feat: add containerized mkdocs-dev instruction

[Shiritai]

Containing a doc_env/Dockerfile to make sure dev env consistency and a description document doc_env/README.md.

Description

This PR adds a containerized development environment for MkDocs in the Openadkit project. It introduces a Dockerfile and a README.md in the doc_env/ directory, providing instructions to build and run a MkDocs development server. The setup replicates the dependencies from the GitHub Actions workflow, ensuring consistency between local and CI/CD environments.

• Added doc_env/Dockerfile to create a Python 3.11-based image with required MkDocs plugins.
• Added doc_env/README.md with detailed instructions for building, running, and troubleshooting the containerized environment.

How was this PR tested?

• Built the Docker image using docker build -f doc_env/Dockerfile -t mkdocs-dev ..
• Ran the development server with docker run -it --rm -p 8000:8000 -v $(pwd):/app mkdocs-dev and verified the MkDocs site loads at http://localhost:8000.
• Generated the static site using docker run -it --rm -v $(pwd):/app mkdocs-dev mkdocs build and confirmed the site/ directory was created correctly.
• Tested with a .dockerignore file to exclude unnecessary directories (e.g., deployments/, .github/).

Notes for reviewers

• Please review the .dockerignore recommendations in doc_env/README.md to ensure optimal build performance.
• The Dockerfile omits COPY instructions for mkdocs.yml and docs/ to avoid hardcoding file paths, relying on volume mounts for flexibility. Confirm if this approach aligns with project needs.

Effects on system behavior

None. This PR only adds documentation and development tools without affecting existing project functionality.

[evshary]

Very detailed explanation. I think the folder name "docker" might be confusing. Users might not know this is only for the documentation environment.
How about using doc_env or something else?