Dev: Add Design System and Style Guidelines section

- See [FPA grant 58: Design System and Style Guidelines](https://github.com/FreeCAD/FPA-grant-proposals/issues/58)

Open questions:
- Everything in `/dev/design`?
- Linear/flat or nested pages structure?
- One page per component or multiple components in a single page?
- [Shortcodes](https://gohugo.io/content-management/shortcodes/) for components are probably the most flexible implementation for the website, but it can become quite complex. Maybe simple SVG files with chroma support (light/dark vars) are sufficient? I would not go the data file route, as it is probably overkill and too much to maintain, even though powerful.
- How much "interaction" do we want? Probably very little or none. Tiny JS or CSS in SVG can be added for enhancement if wanted.
- How should it be exposed in the CMS? Minimal with focus on text content, or full editable components too (probably too much)?
- TBD

Start with the linked FPA grant proposal and the website's /dev/design area. Resolve the open questions about page structure, component grouping, implementation approach, interaction, and CMS exposure before making changes. Done means the design system and style guidelines are defined and integrated into the website structure.

Written by the indexing model from the issue text.