Merge bin/phbbpcli.php and install/phpbbcli.php

If you attempt to install from the command-line instead of the web wizard, and invoke bin/phpbbcli.php instead of install/phpbbcli.php, you get a very cryptic error message. (It works after the installation is complete) This cryptic error message should not appear, instead, the error message should say "run install/phpbbcli.php" instead.

Well, not really. Why should there be two executables doing mostly the same thing with the same name, in two different directories, but behaving differently? Merge them seem to be the right answer.

And while at it, document in the main user documentation, not on some separate static documentation site that may not be actively maintained (https://area51.phpbb.com/docs/dev/3.1.x/cli/index.html)