Add theoretical part to documentation

[paulmueller]

The theoretical part should refer to wikipedia and state how the normalization works. Maybe we should also use the same notation as Wikipedia. Currently, the source uses n as k and N as n.

• computation of correlation from fluctuation of signal
• ergodic system: time average is expectation
• normalization: By signal and n-k, biased
• explicitly state that we are not using the variance for normalization, but the intensity
• motivate this normalization for FCS, as already in the docs; maybe link to this page

[paulmueller]

Should we put the theoretical part into the doc string of multipletau/init.py which is automatocally included in the documentation anyway?