Improve documentation

[timja]

1. Simplified documentation
2. Made updates to instructions based on changes in Jenkins that have happened since it was written
3. Stop using deprecated jnlp endpoint for inbound agents, see Simplifying JNLPLauncher jenkins#8762
4. Move massive groovy section to separate page
5. Document inbound agent in the launcher help file, it was documented in init script but was missed in launcher which I think confused @dduportal in Inbound: support for websockets #421
6. Remove Microsoft support message, its well past the date they stopped 'supporting' the plugin

Fixes #421
Fixes #422

Testing done

Tested the linux inbound steps on a controller in Azure

Stepped through documentation while creating so it should work better

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