refactor: remove open_url config option in favour of :CP open (#322)

## Problem

`open_url` automatically opened the browser on contest load and problem
change, which is now redundant with `:CP open`.

## Solution

Remove the `open_url` field from `cp.Config`, its default, its
validation, and the call site in `setup_contest`. Remove documentation
from `cp.nvim.txt`.

lua/cp/config.lua

@@ -101,7 +101,6 @@
 ---@field templates? CpTemplatesConfig
 ---@field hooks Hooks
 ---@field debug boolean
----@field open_url boolean
 ---@field scrapers string[]
 ---@field filename? fun(contest: string, contest_id: string, problem_id?: string, config: cp.Config, language?: string): string
 ---@field ui CpUI
@@ -118,7 +117,6 @@ local utils = require('cp.utils')
 -- defaults per the new single schema
 ---@type cp.Config
 M.defaults = {
-  open_url = false,
   languages = {
     cpp = {
       extension = 'cc',
@@ -362,7 +360,6 @@ function M.setup(user_config)
     hooks = { cfg.hooks, { 'table' } },
     ui = { cfg.ui, { 'table' } },
     debug = { cfg.debug, { 'boolean', 'nil' }, true },
-    open_url = { cfg.open_url, { 'boolean', 'nil' }, true },
     filename = { cfg.filename, { 'function', 'nil' }, true },
     scrapers = {
       cfg.scrapers,