Expand reactive operator docs with category-based "Manual" articles

[banchan86]

As I was exploring #117 I realized that the different reactive operator groupings are good starting points for additional "Manual" articles to enhance our reactive documentation. This would be similar to how the the current "Subjects" article is structured (a general overview/explanation, followed by coverage of the individual operators). Some issues to consider:

• Do we want to cover all the groups, or only some? Some articles that I def think would be good to spin off:
  • Filtering, selecting or grouping observables
  • Using multiple observable sequences (triggering, combining)
• Where to put them?
  • We can put them under the language guide, but as the table of content grows, it might be better to spin off a new "Reactive" or "Working With Sequences" section to prevent the article TOC from becoming too overwhelming.
  • Some of the current content (especially the "Extend Bonsai" section) can be spun off as a new "Developers" section

I will list down more thoughts as they come to me, and spin off separate issues for the articles as they become more concrete, but for now this will serve as a meta issue for discussion.