Rules: Replace placeholder with ESH text (openhab#893)

* Rules: Replace placeholder with ESH text

This is a first step in improving the NG Rules page.
In a follow up PR this is split up into a technical part and a user part and https://community.openhab.org/t/experimental-next-gen-rules-engine-documentation-2-of-installation/55955 need to be integrated of course.

Signed-off-by: davidgraeff <[email protected]>

* Update rules-ng.md

Fix http->https

* Update configuration/rules-ng.md

Co-Authored-By: davidgraeff <[email protected]>

* Update rules-ng.md

This is no longer a pure copy. I have broken down some super long sentences and reformulated some paragraphs.

* Update rules-ng.md

Incorporate suggested changes and fixed typo