Documentation revision

Many parts of the documentation were outdated. This MR is an attempt to bring the documentation website up to date again.

As outlined in #228, important missing parts concern the following topics (to be followed up in future MRs):

• Framework design, including a description of Gaudi-Allen algorithms
• Short description of all HLT1 lines (API reference exists already, but need to add docstrings to python code)
• One-line description for every algorithms, plus physics cut-offs that are applied
• Description of the Allen event model