[FEATURE]: Document how to set port number to operate on

[brendandebeasi]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When running forge with the default port of 3000 in use the startup fails. I was able to guess the argument to pass port number, but it should be documented.

Additionally, I'd like to know how I can set this as an env var

b@bdm1 ~ % forge --port 3003
2152 |     })
2153 |     return result
2154 |   }
2155 |
2156 |   export function listen(opts: { port: number; hostname: string }) {
2157 |     const server = Bun.serve({
                              ^
error: Failed to start server. Is port 3003 in use?
 syscall: "listen",
   errno: 0,
    code: "EADDRINUSE"

      at listen (src/server/server.ts:2157:24)
      at server (src/cli/cmd/tui/worker.ts:36:23)
      at server (src/cli/cmd/tui/worker.ts:33:16)
      at <anonymous> (src/util/rpc.ts:10:41)
      at onmessage (src/util/rpc.ts:7:24)