Cleanup documentation for 1.0.0

[didier-wenzek]

In its current state, the documentation for 1.0.0-rc.1 still contains some content related to deprecated features and former design. One must at least remove and cleanup what is confusing.

• [x] Remove content related to tedge-configuration-plugin and tedge-log-plugin from the tedge-agent overview. https://github.com/thin-edge/thin-edge.io/pull/2554
• [x] Move the section "Connecting an external device" under "Security and access-control" - https://github.com/thin-edge/thin-edge.io/pull/2560
• [x] Update the section "Use MQTT authentication for local broker" to include certificate-based authentication for HTTPS. https://github.com/thin-edge/thin-edge.io/pull/2585
• [x] Update the section "Configuration management on child-devices" to remove references to c8y-configuration-plugin. https://github.com/thin-edge/thin-edge.io/pull/2593
• [x] Update or deprecate child_device_agent.py and remove reference to this plugin from the section "Configuration management on child-devices" . https://github.com/thin-edge/thin-edge.io/pull/2593
• [x] Remove the section "How to retrieve a JWT (JSON Web Token) to authenticate on Cumulocity" - https://github.com/thin-edge/thin-edge.io/pull/2558
• [x] Move the section "Device Firmware Management using Cumulocity" under "Extend/Firmware" with an explicit note that this is the legacy API. - https://github.com/thin-edge/thin-edge.io/pull/2581
• [x] Consider to add a "Legacy / Porting to 1.0" section telling how to move from the tedge to the te topics, how to port a custom log/config/firmware plugin from the req/res model to the cmd model. - https://github.com/thin-edge/thin-edge.io/pull/2581
• [x] Move "Enable configuration management on child devices" under a "Legacy" section. - https://github.com/thin-edge/thin-edge.io/pull/2581
• [x] Update the section "thin-edge.io platform support" with accurate numbers and supported platforms. https://github.com/thin-edge/thin-edge.io/pull/2594
• [x] Remove the section "thin-edge Domain Model" - https://github.com/thin-edge/thin-edge.io/pull/2559

[gligorisaev]

QA has thoroughly checked the feature and here are the results:

• [ ] Test for ticket exists in the test suite.
• [x] QA has tested the function and it's functioning according description.