fix(docs): make VitePress base path configurable for deploys

[ottobolyos]

Summary

Sets the VitePress base from a DOCS_BASE environment variable (defaulting to /), wires the docs workflow to pass DOCS_BASE: /MTConnect.NET/ at build time, and adds a contributor-facing page that documents the docs-site build and the new variable. Fixes the rendering issue on https://trakhound.github.io/MTConnect.NET/ where the deployed page loaded without CSS or JavaScript because asset URLs were absolute-from-domain-root (/assets/...) instead of project-prefixed (/MTConnect.NET/assets/...).

• docs/.vitepress/config.ts: replace the commented-out base line with base: process.env.DOCS_BASE ?? '/'. Local vitepress dev and vitepress build keep the bare-root behavior when DOCS_BASE is unset; CI and other deployment targets opt in by setting the variable.
• .github/workflows/docs.yml: pass DOCS_BASE: /MTConnect.NET/ on the VitePress build step so the GitHub Pages deploy receives correctly-prefixed asset URLs.
• docs/development/docs-site.md: new contributor-facing page covering local dev, the production build, the DOCS_BASE environment variable (with a four-row table mapping deploy URLs to base values), how CI sets the base, and the diagnostic symptoms of a base mismatch. Wired into the /development/ sidebar in docs/.vitepress/config.ts.