docs: add how to customize SecureHeaders
docs: add how to customize SecureHeaders

lonnieezell/CodeIgniter4,bcit-ci/CodeIgniter4,kenjis/CodeIgniter4,samsonasik/CodeIgniter4,lonnieezell/CodeIgniter4,samsonasik/CodeIgniter4,lonnieezell/CodeIgniter4,kenjis/CodeIgniter4,kenjis/CodeIgniter4,bcit-ci/CodeIgniter4,kenjis/CodeIgniter4,kenjis/CodeIgniter4,lonnieezell/CodeIgniter4,samsonasik/CodeIgniter4,bcit-ci/CodeIgniter4,lonnieezell/CodeIgniter4,samsonasik/CodeIgniter4,lonnieezell/CodeIgniter4,samsonasik/CodeIgniter4,bcit-ci/CodeIgniter4,samsonasik/CodeIgniter4,bcit-ci/CodeIgniter4