feat: article and configuration file example about stunnel

examples/stunnel.config

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+[redis-cli]
+client = yes
+accept = 127.0.0.1:6380
+connect = redis.lan:6380
+
+; CAfile = /home/user/trusted.certs.pem

knowledge base/stunnel.md

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+# Stunnel
+
+Proxy designed to add TLS encryption functionality to existing clients and servers without any changes in the programs' code.
+
+## Table of contents <!-- omit in toc -->
+
+1. [TL;DR](#tldr)
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## TL;DR
+
+In Unix-like operating systems:
+
+```sh
+# Install it.
+brew install 'stunnel'
+
+# Show default socket options.
+stunnel -sockets
+
+# Show supported TLS options.
+stunnel -options
+
+# Start the process.
+stunnel 'path/to/config/file'
+```
+
+## Further readings
+
+- [Website]
+- [Sample configuration for Unix systems]
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+<!-- project's references -->
+[sample configuration for unix systems]: https://www.stunnel.org/config_unix.html
+[website]: https://www.stunnel.org/
+
+<!-- in-article references -->
+[further readings]: #further-readings
+
+<!-- internal references -->
+<!-- external references -->