Documentation versions

[rondiplomatico]

Like with CMake itself, there are records of documentations for each released version - i'd suggest we put something like this in place for the documentation as well.
two possible structures are obvious:

• have a global version of the documentation. i suggest we identify that with the opencmiss release version 'stored' in opencmiss/manage
• have no 'top-level' version but have versions for each component, meaning "manage, iron, zinc" and possibly others.

i would take the first approach.