[DOCUMENTATION] Three books instead of only one

The User one is intended to illustrate the various common use cases, possibilities regarding customization and introduce the existing functionalities.

The 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 Developer one 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.

docs/developer/src/installation.md

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+# Installation
+
+GNU social is intended to be easily installable in both a shared hosting environment or a private
+host with shell access, or just with PHP execution.
+
+If you need help, contact us on IRC on the `#social` room in freenode or XMPP at [xmpp:gnusocial@conference.bka.li](xmpp:gnusocial@conference.bka.li)
+
+The recommended way of installing is to use [Docker](https://www.docker.com/), as this simplifies
+configuration. GNU social is comprised of a variety of different services, such as a webserver, a
+PHP execution environment, a database, etc. You may choose to use all, some, or none of these
+services in Docker containers.
+
+Pick one of the following installation methods:
+
+ - [Instal with Docker with shell access](./install/docker_shell.md)
+ - [Instal without Docker with shell access](./install/no_docker_shell.md)
+ - [Instal with Docker with web access](./install/docker_web.md) (requires access to PHP's `system()`, which may be disabled)
+ - [Instal without Docker with only web access](./install/no_docker_web.md)
+
+Installation with Docker without shell access, such as in some shared hosting environments is
+possible by configuring social locally and copying the files over, however this is not a supported
+configuration.