Clean up 'images' documentation

Clean up 'images' section documentation by moving it into a
sub-section of 'providers', increase verbosity of descriptions around
name choice and flavor choice, document the previously undocumented
name-filter and organise flow to empahsise important points first.

Change-Id: I27c5fc9676a3fd342f4121bf539c3c81b5830ff2
Merge "Clean up 'images' documentation"