Unify project documentation

[Nxssie]

Description:
Documentation for nan is currently fragmented and out of sync across multiple repositories (e.g. helmcode/nan, helmcode/nan-discord-bot, etc.). This makes it hard to maintain, leads to inconsistencies, and creates a poor onboarding experience.

Goal:
Consolidate all documentation into a single source of truth.

Scope:

• Audit existing docs across all related repositories.
• Identify duplicated, outdated, or conflicting content.
• Define the canonical location for unified documentation.
• Migrate and reconcile content into the chosen location.
• Update references/links in the other repos to point to the unified docs.

Acceptance criteria:

• A single, up-to-date documentation source exists.
• No conflicting/duplicated docs remain in secondary repos.
• All repos link back to the unified documentation.

[barckcode]

Closing as duplicate of #3, which contains the concrete design (Astro Content Collections + /api/docs/manifest.json + /api/docs/{slug}.md) and the migration plan. #3 will be closed automatically when #4 is merged. If documentation duplication shows up in other repos (e.g. nan-cli, nan-devops), let's open a specific issue per repo rather than reopen this umbrella.