Missing documentation how to connect to an existing blockchain network SP 1

[Stezido]

• I have searched the issues of this repository and believe that this is not a duplicate.

Description 😯

In the official documenation of Trubudget there is an old version describing how to connect to a blockchain spinning up a slave node https://trubudget.net/docs/operation-administration/installation/connect-to-existing-node/connect-docker-compose

In the github documentation there is no documentation how to connect to a blockchain spinning up a slave node but there is a hidden section in the "create a new network" section: https://github.com/openkfw/TruBudget/blob/master/docs/operation-administration/installation/create-new-network/create-new-docker-compose.md#connect-to-an-existing-blockchain-network
This docu is NOT describing how to connect to an existing blockchain network but it's describing how to spin up a new network with 2 organizations connected.

The goal is to have 2 documentation sections synced with the Trubudget website and the github docu:

1. Create a new network
2. Connect to an existing network

Also the misleading doc section in "Create a new network" should be renamed to something like "Create a new network with 2 organizations".

[stale]

This issue has been automatically marked as stale because it has not had activity for 30 days. It will be closed if no further activity occurs. Thank you for your contributions.

[Stezido]

we should also rethink our operation/dev environment and its documentation.
For devs it's docker-compose scripts are more useful and for external admins/operations the scripts would make more sense.

[Stezido]

The .env_example file should not be used in any automated startup script.