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/install/docker.md

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+# Docker
+
+You can quickly get a server running using Docker. You'll need both [Docker](https://www.docker.com/community-edition) and [Docker-Compose](https://docs.docker.com/compose/install/).
+
+Start by cloning the repo
+```bash
+git clone https://framagit.org/framasoft/mobilizon && cd mobilizon
+```
+
+Then, just run `make` to build containers.
+```bash
+make
+```
+
+This will start a database container, an API container and the front-end container running on localhost.