Remove embedded documentation's calls to the update-center

WHY

Currently, the embedded doc is fetching information on scanners from the update-center (release notes, download urls, etc). This is a security concern and we want to avoid such calls as much as possible.

WHAT

We should drop the scanner download component from scanner's documentation page within the embedded doc (only), and replace them with links to redirect.sonarsource.com.

HOW

1. Drop support of the "<update-center />" tag in the embedded documentation
2. Replace every reference to the upper mentioned tag in scanner's page by a download section: Download - To download the latest version of the scanner, see the online documentation. (Link behind "online documentation")