update documentation: make clear client and server are now separate

[wivaku]

Related to #1 , it would help to:

• fix the Open on Github link (tooltip: "Contribute to Expose Server on Github") - link should open https://github.com/exposedev/server
• on https://expose.dev/docs/server/server/starting-the-server remove the sentence:
  "The expose binary that you install via composer contains both the server and the client, so you do not need any additional software for this to work."
  and replace with instructions how to install the server binary
• in the Expose v3 release notes (https://expose.dev/docs/release-notes) mention that client and server are no longer the same binary, and that client v3 requires server v3.