Update docsy and tooling

[kreativmonkey]

This PR updates all tooling used in the project (Hugo, Node.js, Go modules) and introduces a Nix flake-based development environment for easier local development.

Changes

1. Updated Docker Base Image

• Switched from klakegg/hugo to ghcr.io/hugomods/hugo for the Docker build
• Added npm ci step to properly install npm dependencies during Docker build
• Updated Hugo from 0.111.3 to 0.154.5

2. Added Nix Flake Development Environment

• Created flake.nix with development shell
• Includes hugo and nodejs_20 as build inputs
• Provides a check-build helper function for building and validation

3. Updated Go Modules

• Updated docsy theme from v0.4.0 to v0.14.3
• Updated docsy/dependencies from v0.4.0 to v0.7.2
• Switched from acend/docsy-plus to kreativmonkey/docsy-plus (fork)
• Updated docsy-puzzle references to latest commits

4. Updated NPM Packages

• Added postcss, postcss-cli, and autoprefixer as dependencies (required by Hugo)
• Updated markdownlint-cli from 0.32.2 to ^0.48.0
• Updated package.json start script to use new Docker image

5. Improved .gitignore

• Added extensive patterns for Hugo cache, node_modules, and build artifacts

Breaking Changes

This is a breaking change, the update from docsy 0.4.0 to 0.14.3 has a lot of breaking changes included and it will only work if the forked version of docsy-plus is used as well as the pull-request puzzle/docsy-puzzle#2 for docsy-puzzle is merged. To make future upgrades a bit more easy, please use tags for docsy-puzzle as used for the forked version of docsy-plus.

[github-actions]

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