docs/configuration: Fix entropy for bash secret

Filtering `/dev/urandom` for alphanumeric characters resulted in loss of
input entropy to base64. Fixing this using a procedure with these steps:

  * Read 32 bytes from `/dev/urandom` (`dd`)
  * Base64-encode (`base64`)
  * Strip newlines (`tr -d`)
  * URL-Escape (`tr`)
  * Append a final newline (`echo`)

This output should be equivalent to output generated using Python and
OpenSSL variants mentioned in the changed document file.

Newlines are stripped as `base64` wraps its output and the option to
disable this (`-w 0`) is not available in all implementations.

Fixes: #1511