Better docs

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Nicer docs

No 3rd stage

Add mix.deps get before docs

Add :ex_doc on test env so that it runs into CI

support/guides/development/development.md

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+# Development
+Clone the repo, and start the project trough Docker. You'll need both Docker and Docker-Compose.
+```bash
+git clone https://framagit.org/framasoft/mobilizon && cd mobilizon
+make
+```
+## Manual
+
+### Server
+
+  * Install dependencies:
+    
+    * Elixir (and Erlang) by following the instructions at [https://elixir-lang.github.io/install.html](https://elixir-lang.github.io/install.html)
+    * Fetch backend Elixir dependencies with `mix deps.get`.
+    * [PostgreSQL]() with PostGIS
+  * Start services:
+    * Start postgres
+  * Setup services:
+    * Make sure the postgis extension is installed on your system.
+    * Create a postgres user with database creation capabilities, using the
+      following: `createuser -d -P mobilizon` and set `mobilizon` as the password.
+    * Create your database with `mix ecto.create`.
+    * Create the postgis extension on the database with a postgres user that has
+      superuser capabilities: `psql mobilizon_dev`
+
+      ``` create extension if not exists postgis; ```
+
+    * Run migrations: `mix ecto.migrate`.
+  * Start Phoenix endpoint with `mix phx.server`.
+
+Now you can visit [`localhost:4000`](http://localhost:4000) from your browser
+and see the website (server *and* client) in action.
+
+### Client
+
+If you plan to specifically change the client side (frontend), do the following
+once the server is running:
+
+  * Install the NodeJS (we guarantee support for the latest LTS and later) ![](https://img.shields.io/badge/node-%3E%3D%2010.0+-brightgreen.svg)
+  * Change directory to `js/` and do:
+    * Install JavaScript package dependencies: `npm install`.
+    * Run the developement server in watch mode: `npm run dev`. This will open a
+      browser on [`localhost:8080`](http://localhost:8080) that gets
+      automatically reloaded on change.
+
+## Docker
+You need to install the latest supported [Docker](https://docs.docker.com/install/#supported-platforms) and [Docker-Compose](https://docs.docker.com/compose/install/) before using the Docker way of installing Mobilizon.
+
+Just run :
+```bash
+make start
+```
+to start a database container, an API container and the front-end dev container running on localhost.