Integrate the content from the wiki which is still relevant

[pcdavid]

We used to maintain a set of wiki pages on the Eclipse Foundation's wiki, but this wiki instance is shutting down.

A lot of these pages are old and contain out of date or irrelevant information, but there are some with content that is still relevant/useful and should be re-integrated, either in the documentation or in the top-level project files (enabling the GitHub repo's wiki just for these few pages seems overkill).

The pages with still relevant information (at least partially) are:

• Contributor Guide: should be updated and integrated into the top-level CONTRIBUTING.md.
• FAQ and Cookbook: probably into a new section in the documentation.
• Releng: should be updated/completed and put into a new top-level RELENG.md
• Update Sites: converted into a table and put into a new top-level RELEASES.md

For reference, a "dump" of the full text of all the current wiki pages should also be committed somewhere, probably in org.eclipse.sirius.doc.