docs: add repository-level Copilot instructions (#105)

This PR sets up `copilot-instructions.md` for the repository by adding a
concise, repo-specific instruction file. The goal is to give Copilot
immediate context on how this codebase is organized and operated.

- **Added Copilot instruction baseline**
- Created `.github/copilot-instructions.md` with focused guidance for
this Astro + Starlight docs repository.

- **Documented high-signal developer commands**
- Added the canonical root-level commands used for local development and
validation:
    - `npm install`
    - `npm run check`
    - `npm run build`
    - `npm run dev`

- **Captured content conventions Copilot should follow**
  - Documented docs content location: `src/content/docs/`
  - Documented news author source: `src/content/docs/news/authors.ts`
- Documented required news frontmatter fields and `slug` prefix rule
(`news/`)

```md
# Copilot Instructions for `trueforge-org/website`

## Development commands
- `npm install`
- `npm run check`
- `npm run build`
- `npm run dev`

## Content conventions
- Documentation content lives in `src/content/docs/`.
- News posts must include `title`, `slug` (starting with `news/`), `date`, and `authors`.
```

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `telemetry.astro.build`
> - Triggering command:
`/home/REDACTED/work/_temp/ghcca-node/node/bin/node node
/home/REDACTED/work/website/website/node_modules/.bin/astro build` (dns
block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/trueforge-org/website/settings/copilot/coding_agent)
(admins only)
>
> </details>

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/trueforge-org/website/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
your repo.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: PrivatePuffin <[email protected]>

Author: Copilot

.github/copilot-instructions.md

@@ -0,0 +1,18 @@
+# Copilot Instructions for `trueforge-org/website`
+
+This repository is an Astro + Starlight documentation site.
+
+## Development commands
+
+Run all commands from the repository root:
+
+- `npm install`
+- `npm run check`
+- `npm run build`
+- `npm run dev`
+
+## Content conventions
+
+- Documentation content lives in `src/content/docs/`.
+- News authors are defined in `src/content/docs/news/authors.ts`.
+- News posts must include `title`, `slug` (starting with `news/`), `date`, and `authors` in frontmatter.