Restructuring the user documentation

I am proposing certain changes in the ordering and structuring of the sections within the 'user documentation' to give it a better 'flow of information' and continuity.

Please see [this](https://docs.google.com/document/d/16fmgzT8iFgFq6KtL9Rzr6X3htXdfCQ2l1wfwDP8mUFw/edit?usp=sharing) google doc.

**Proposed changes:**

- [X] Reorder the subsections within the user docs, to have the most important subsections on top. (completed in #1455 )
- [ ] Take the 'what is conda-forge' section out of 'a brief intro' and put it at the beginning of the documentation. (see why [here](https://docs.google.com/document/d/16fmgzT8iFgFq6KtL9Rzr6X3htXdfCQ2l1wfwDP8mUFw/edit#heading=h.vkcwv1j5vlr9))
- [ ] Split 'why conda-forge' into 'what does conda-forge offer' and 'brief history'. (see why [here](https://docs.google.com/document/d/16fmgzT8iFgFq6KtL9Rzr6X3htXdfCQ2l1wfwDP8mUFw/edit#heading=h.9ask9n3xrovl))
- [x] Add 'can I contribute packages to conda-forge?' section. (see why [here](https://docs.google.com/document/d/16fmgzT8iFgFq6KtL9Rzr6X3htXdfCQ2l1wfwDP8mUFw/edit#heading=h.6bp9gsyqbos5)) (completed in #1455)