docs: add a public documentation site for installation and user guides

[xoriuz]

Problem

Termul does not have a hosted, user-oriented documentation site.

The landing page currently sends users to the repository's docs/ directory. That directory mainly contains internal architecture, API contract, deployment, and contributor material. Installation guidance lives inside the long root README, and there is no focused onboarding path for new users.

Proposed solution

Add a standalone public documentation site using Fumadocs and React Router.

Proposed deployment:

• Landing: https://termul.dev
• Documentation: https://docs.termul.dev
• Hosting: separate Cloudflare Pages project
• Content: MDX stored in website-docs/
• Initial page: installation and first project workspace setup

The existing docs/ directory would remain the internal architecture and contributor knowledge base.

Initial scope

• Fumadocs documentation shell
• Static prerendering
• Local search
• Installation guide for Windows, macOS, and Linux
• First workspace instructions
• Docs-specific lint, typecheck, and build commands
• Cloudflare Pages deployment workflow
• Landing integration only after the docs domain is live

Infrastructure requirement

A maintainer must create or confirm:

• Cloudflare Pages project termul-docs
• custom domain docs.termul.dev
• deployment access through the repository's Cloudflare secrets

Out of scope

• Rewriting the internal architecture documents
• Moving docs/ into the public site
• Documenting every feature in the initial pull request
• Redesigning the landing page

Acceptance criteria

• Documentation app builds as static output
• /docs renders an installation guide
• Search works
• Direct route refreshes do not return a 404
• CI validates docs lint, types, and build
• Cloudflare deployment requirements are documented
• Landing link changes only after docs.termul.dev is ready

Contribution

I can contribute the initial Fumadocs shell and installation guide if this deployment structure is acceptable.