WL#5300: Reorganise MySQL Cluster installation information

Reorganise Cluster installation information (currently called 'MySQL Cluster
Multi-Computer How-To') along platform-centric lines. Reorganise Linux
installation by method (binary, RPM, source).

Get rid of references to 'Multi-Computer' and 'How-To'.

Improve Upgrade/Downgrade section. Sort info by release series. Get rid of
charts + bulleted lists and replace with per-series tables containing info by
release.