Add back a top level docs/technical-reference/architecture page to prevent broken links

[ostephens]

Following the addition of the v4 architecture documentation, the original architecture documentation was renamed to architecture-before-4 which means that links to the old architecture URL give a 404

I propose adding back a page at docs/technical-reference/architecture which simply points to each of the new documents. The page does not need to be part of the sidebar navigation, but just to prevent 404 from existing links to the OpenRefine docs

[aakash-a-dev]

Hello I raised a Pull Request on this issue. Will like you to see this once https://deploy-preview-272--openrefine-website.netlify.app/docs/technical-reference/architecture
This is added on the best of my idea, I am open to feedback if any.