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

docs/docs/configuration/overview.md

@@ -31,7 +31,7 @@ import TabItem from '@theme/TabItem';
   <TabItem value="bash">
 
   ```shell
-  cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 1 | base64
+  dd if=/dev/urandom bs=32 count=1 2>/dev/null | base64 | tr -d -- '\n' | tr -- '+/' '-_'; echo
   ```
 
   </TabItem>