update readmes

[casperdcl]

• drop outdated Gadgetron instructions (fixes #833)
• update install (docker, VM, CMake, etc.) & examples info (fixes #871)
• misc formatting updates
• ~re-enable CCPi-Regularisation~ moved to #832

[KrisThielemans]

@paskino @casperdcl what's the status of this one?

[casperdcl]

going to split this out into separate PRs

[casperdcl]

@KrisThielemans ready for review :)