I believe very strongly in the power of documentation, for a number of reasons. Knowledge should be kept in an ordered manner, and writing documentation is a good way of not only ordering the knowledge, but double-checking it. In addition, by placing the documentation somewhere accessible, you can share it.
What platform you choose depends on what your goal is. If you need the documentation to be available in the event of an apocalyptic event, using a digital platform is likely not a great solution, and you will probably want to resort to a three-ring binder. That, of course, opens its own can of worms, with the need to update all copies, lack of historic data and so on.
Leaving that aside, however, the platform you choose will be dictated by your needs. If it is your personal documentation, which is only relevant to you, OneNote (or a similar solution) will likely suffice. A blog can work well, but information tends to drown in a blog, and you will likely need to rely on tags and searches.
A collaboration platform such as Atlassian Confluence brings powerful tools such as access control at both the system and space levels, markup tools and vast options for plugins. If you have a fully fledged service management suite, there are likely tools built in to that which will help you out.
At any rate, all of this is to say that thinking about this, making a decision, and then following it through is very important. In my opinion, more important than what platform you choose, is to ensure that it has good search capabilities. Sure, being able to sort into categories is very nice, but it is often more useful and productive to search instead.