What you seem to have in mind is a monstrously big project. If you want it to succeed, you'd need a panel of specialists ready to stay on for the duration of the work and someone with a good mind and clear head chairing the panel and keeping an eye not only on contents but also on rights(!), accessability, structure etc. Plus: money, an editor or a group of editors, money, someone for the final editing, money, web space, money, someone with the expertise to handle the technical side of things, and the certainty that the manual will be taken care of every few weeks for a number of years and taken offline as soon as this can't be guaranteed any longer. Poorly cared for online databases are a nuisance and a hindrance.
Just to assemble the first version will take at least a year ot two, provided several people put in some hours of work every day.
Sorry to put such a damper on your dreams and hopes, but please let me tell you that this can not possibly be done on the side. The amount of work involved is amazing, and it tends to get more and more as you go along.