docs(server): rename self-hosting server docs

What changed

• Renamed the self-hosting page title and heading from “Control plane” to “Server”.
• Updated the self-hosting sidebar label from “Control plane” to “Server”.
• Updated the English docs navigation string to match the new server label.
• Moved the canonical page slug from /guides/server/self-host/control-plane to /guides/server/self-host/server.
• Added a redirect from the old /control-plane slug and retargeted legacy self-hosting redirects to the new /server slug.

Why

The page documents deploying and configuring the self-hosted Tuist server. “Control plane” was technically adjacent to the cache/server architecture, but it was more abstract than the terminology we use with users and internally. “Server” makes the self-hosting docs easier to scan while keeping the existing “Cache” label unchanged.

Root cause

The self-hosted server guide had inherited the control-plane wording while the actual content describes the Tuist server deployment. That made the page title feel disconnected from the content.

Impact

Users see clearer labels in the docs sidebar and page metadata. The canonical self-hosted server guide now lives at /guides/server/self-host/server, while the old /guides/server/self-host/control-plane URL redirects to it for link compatibility.

Validation

• jq empty server/priv/docs/strings/en.json
• git diff --check HEAD~1 HEAD
• /Users/marekfort/.local/share/mise/installs/elixir/1.19.5/bin/mix format lib/tuist/docs/redirects.ex lib/tuist/docs_sidebar.ex test/tuist/docs/redirects_test.exs
• /Users/marekfort/.local/share/mise/installs/elixir/1.19.5/bin/mix test test/tuist/docs/redirects_test.exs