add subsections inside community section

[OriolAbril]

• important background, or details about the implementation

  • Extends the community section to make it easier for other contributors to add
    more documentation about it.

    In general, all sections but the "contributing" one have pymc users as intended audience,
    things that have contributors as expected audience so far have gone inside the
    "contributing" section (so "how to set up office hours" page would go there).
    We can change that and add its own contributing inside the community section
    but I think it is good to have the guides of all types of contributions
    in the same place so that hosting office hours, building docs or writing code are
    perceived as similar and same level contributions.

cc @cluhmann @reshamas