[DOCS][Developer] Add an introduction

docs/developer/src/SUMMARY.md

@@ -1,5 +1,6 @@
 # Summary
 
+- [Introduction](./introduction.md)
 - [Architecture](./architecture.md)
 - [Programming Style](./paradigms.md)
 - [Exceptions](./exceptions.md)

docs/developer/src/introduction.md

@@ -0,0 +1,27 @@
+# GNU social Developer's handbook
+
+The book in your hands was written to teach you
+the art of contributing to the GNU social codebase.
+
+It starts by introducing the Modules system and architecture,
+then the plugin development process and finally the exciting internals
+of GNU social for those looking forward to make the most advanced contributions.
+
+# What you need to dive in
+
+At least a webserver such as [nginx](https://nginx.org/) and a DBMS such as [postgresql](https://www.postgresql.org/).
+
+Depending on what you want to do, you may want to setup a queues (and caching) system such as [redis](https://redis.io/).
+
+To learn how to set all of that up, you may refer to the [System Administrator's handbook](/administrator).
+
+Understand that this book assumes prior programming knowledge, if that doesn't sound like you, refer to our [study
+resources](https://www.gnusocial.rocks/soc/study_resources.html) to get started.
+
+# The other documentations are equally relevant to a developer
+
+The [User](/user) one is intended to illustrate the various common use cases, possibilities regarding customization and introduce the existing functionalities.
+
+The [Administrator](/administrator) one explains the step by step of how to install and maintain a GNU social instance, be it as node of The Free Network or as an intranet social network in a company setting.
+
+The [Designer](/designer) one is an in-depth overview of the design motifs, and the design language used. Useful for creating new plugins and help as a basis for new themes.