Documentation for self-hosting with docker compose (#855)

## Purpose

Make self hosting of Docs easier with an example of a deployment
procedure with docker compose and document how to configure Docs.

While https://github.com/suitenumerique/docs/pull/583 propose an easy
way to deploy Docs with docker and Make, here we describe more in
details the various steps and requirements to deploy Docs.

## Proposal

- [x] example to deploy and configure keycloak
- [x] example to deploy and configure minio
- [x] example to configure proxy and certs
- [x] example to deploy and configure Docs

## Improvements
- [x] Rephrase description of environment variables and categorize
- [x] Use template for nginx conf  

Fixes https://github.com/suitenumerique/docs/issues/561
Supersedes https://github.com/suitenumerique/docs/pull/583

 A one liner quick start could be a nice addition:
- [ ] merge all services in a single compose
- [ ] scripts to generate secrets

Signed-off-by: unteem <timothee@indie.host>

env.d/production.dist/keycloak

@@ -0,0 +1,8 @@
+# Keycloak admin user
+KC_BOOTSTRAP_ADMIN_USERNAME=admin
+KC_BOOTSTRAP_ADMIN_PASSWORD=<generate your password>
+
+# Keycloak configuration
+KC_HOSTNAME=https://id.yourdomain.tld # Change with your own URL
+KC_PROXY_HEADERS=xforwarded # in this example we are running behind an nginx proxy
+KC_HTTP_ENABLED=true # in this example we are running behind an nginx proxy