Add front-end doc links in subscription API docs

[e-pettersson-ericsson]

Description

From EI back-end there are no links to front-end docs of how to create subscriptions. The other way around works fine, but from back-end to front-end the links are scarce. In front-end we have more detailed information about subscriptions requirements and conditions and how they relate to each other. However, this information is not easy to find from back-end at the moment.

Motivation

We need to have a simple way of navigating between front-end and back-end docs.

Exemplification

https://github.com/eiffel-community/eiffel-intelligence/blob/master/wiki/markdown/subscription-API.md needs a link to the front-end docs about making HTTP POST requests towards the subscription API: https://github.com/eiffel-community/eiffel-intelligence-frontend/blob/master/wiki/markdown/curl-examples.md#subscriptions

Benefits

Easily accessible documentation about how to create subscriptions towards Eiffel Intelligence (both using the GUI and performing HTTP requests)

Possible Drawbacks

I guess the links would have to point to latest on master when referencing to another repository - which can cause some issues if users are using an older version.

[e-pettersson-ericsson]

Since the changes have been merged I'm closing this issue.