Add tutorials part

doc/markdown/tutorials/index.md

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+# Rspamd tutorials
+
+In this section, you can find step-by-step tutorials for various things related to rspamd.
+
+Currently, there are the following tutorials ready:
+
+* [Writing rspamd rules](writing_rules.md) - here you can find information of how to write new rspamd rules
+* [Training rspamd with dovecot antispam plugin, part 1](https://kaworu.ch/blog/2014/03/25/dovecot-antispam-with-rspamd/) - this tutorial describes how to learn rspamd automatically using `antispam` pluging of `dovecot` IMAP server
+* [Training rspamd with dovecot antispam plugin, part 2](https://kaworu.ch/blog/2015/10/12/dovecot-antispam-with-rspamd-part2/) - the continuation of the previous tutorial