docs: update with open_url option

doc/cp.nvim.txt

@@ -142,6 +142,7 @@ Here's an example configuration with lazy.nvim:
             default_language  = 'cpp',
           },
         },
+        open_url = true,
         debug = false,
         ui = {
           run_panel = {
@@ -210,6 +211,7 @@ run CSES problems with Rust using the single schema:
                         Should return full filename with extension.
                         (default: concatenates contest_id and problem_id, lowercased)
         {ui}            (|CpUI|) UI settings: run panel, diff backend, picker.
+        {open_url}      (boolean) Open the contest url in the browser.
 
                                                              *CpPlatform*
     Fields: ~