Create general design guidelines

Create a document with style principles that apply to the whole project or several component types (ex. General layout principles, colour palette, UI component semantics).
* This helps create a cohesive and standardized tool.
* Guidelines can be adapted and improved as the project evolves and expertise is honed.
* (Guidelines specifically for graphs can be in a separate place.)

Prerequisite: Create chart style guidelines #152 before diving into defining these more abstract guidelines.

# Inspiration
Style guidelines can be strongly inspired by the VS Code and Grafana style either by:
* Finding (or requesting) the actual style guidelines for these tools (and adapting them as needed)
* Reverse engineering guidelines based on observation of these tools


If reverse engineering guidelines, the tag colours in the [Grafana live demo](https://play.grafana.org/) dashboard management could be a good place to get an idea of the colour palette used:


# Content
Style guidelines could be structured as:
* **Checklist:** Concise, well structured, quick to refer to. The main working tool for people who are familiar with the guidelines.
* **Accompanying document:** More detailed expression of certain checklist items (context, theoretical background, etc.) Read by those new to the guidelines or for reference as needed.


**Include good and bad examples** to illustrate the principles. Example pictures are often best (rather than text explanations).

**Could also include subjective or human checks** (ex. Does it feel visually intimidating? Do you feel invited to explore?). High-level user-focused questions can act as sanity checks. They can help avoid tunnel vision by encouraging devs to think about the project from a different perspective.

Start with the prerequisite chart style guidelines issue #152, then review the VS Code and Grafana examples and the linked Grafana live demo for reusable principles. Done means documenting project-wide style principles as a concise checklist with an accompanying explanation, good and bad examples, and any human-focused checks.

Written by the indexing model from the issue text.