Provide more complete documentation

The documentation should live in a place reachable also for non-LHCb users, so that future use cases of Allen outside of LHCb have access.

The first step is to gather (and update if necessary) all information from the existing readme's in one place, probably similarly to the Moore documentation.

List of topics that should be covered by the documentation in addition to existing readmes:

Allen framework related:

• Instructions on how to add tests (see #203 (closed) )
• Information about CMake options and backend interface (see #191 (closed))
• Description of framework's overall design, including Gaudi-Allen algorithms
• Rules that ensure correct execution on CPUs (i.e. single-threaded call of a kernel)

LHCb related:

• Short description of all implemented lines (see #175)
• One-line description for every algorithms, plus physics cut-offs that are applied
• Description of the Allen event model
• Instructions on how to run a validation algorithm on reconstructed objects