DOC: consolidate developer docs in main docs
There is a new page in the docs 'Contributing to pandas', with at the moment the docs on contributing the documentation. And in #6207 I added already some links to various pages.

At the moment, the information for contributors is a bit scattered:
- The developer pages on the website (http://pandas.pydata.org/developers.html)
- The new page on building the docs (http://pandas.pydata.org/pandas-docs/dev/contributing.html)
- Guidelines on bug reports and pull requests in CONTRIBUTING.md (https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md)
- Some things in the wiki (which I see @y-p cleaned up, nice!):
  - extra tips on using git (https://github.com/pydata/pandas/wiki/Using-Git)
  - another contributing page (https://github.com/pydata/pandas/wiki/Contributing) 
  - and other .. (https://github.com/pydata/pandas/wiki)

So maybe we should make a choice where do we want to consolidate the docs:
- We could opt to put everything in the wiki and CONTRIBUTING.md, so further expanding the wiki.
- Or put everything in the main docs
- Or we could try to make a (clear) division:
  - main docs: starting point for newcomers who want to contribute to pandas (how to submit an issue, guidelines for a pull request, basic info on git and building pandas, building the docs, ...)
    - So this would mainly be to somehow merge CONTRIBUTING.md, developer pages on website and contributing page on wiki.
  - wiki: more info for the core developers/regular contributors: some more advanced tips and tricks, release workflow, how to handle the python2/3 issues, which exceptions to raise, ...)
