General technical updates and changes

Following the documentation workshop this merge request contains general non-content updates and changes to the docs:

• Added HIFIS logo
• Added HIFIS favicon
• Set HIFIS color as theme color
• Use fixed-width layout for better control over the layout (if content is too wide it's too complicated anyway)
• Fix indentation in mkdocs.yaml
• Show legal links on every page instead of only the landing page
• Add devcontainer files for local editing
• Add instruction on how to use devcontainer files
• Add local LanguageTool to devcontainer for style checking
• Enable instant navigation