Improve Docker-Compose Install Experience (#9781)

This PR updates the docker-compose installation documentation and env
sample to improve the setup experience.

- Updates the URLs for raw files to reference main branch, which is
likely where new users will be pulling from initially. This seems to be
the most straightforward option; assume that advanced users who want to
retrieve it from a particular tag will know to change the URL for their
scenario.
- Fixes an improperly stated curl command.
- Adds a note that the PGPASSWORD_SUPERUSER should be URL-safe. This is
required since the value is later concat into a PG_DATABASE_URL as a
URL, and expected to be in proper URL format. Touches on #8597.

---------

Co-authored-by: Félix Malfait <felix@twenty.com>
Co-authored-by: Félix Malfait <felix.malfait@gmail.com>

packages/twenty-docker/.env.example

@@ -1,8 +1,9 @@
 TAG=latest
 
-#PGUSER_SUPERUSER=postgres
-#PGPASSWORD_SUPERUSER=replace_me_with_a_strong_password
+#PG_DATABASE_USER=postgres
+#PG_DATABASE_PASSWORD=replace_me_with_a_strong_password_without_special_characters
 #PG_DATABASE_HOST=db
+#PG_DATABASE_PORT=5432
 #REDIS_URL=redis://redis:6379
 
 SERVER_URL=http://localhost:3000