Duplicate sections in the documentation site

[ZviBaratz]

The README update in #1195 included an installation section which wasn't there before and creates a redundancy in the documentation site.
Looking at the download and installation instructions page, my impression is that it might be possible to remove that page altogether. There might be a couple of bits which could be ported to other locations, but nothing crucial. WDYT?

[ZviBaratz]

Actually, the "License" and "License reprise" sections, as well as the "Mailing list" and "Support" sections, also seem redundant. Definitely seems like they should be merged.

[ZviBaratz]

@matthew-brett @effigies do you agree the download and installation page in the docs isn't really required anymore? Are there any particular parts of it you would like to see ported to the README?

[effigies]

I actually think the detailed download and installation page is useful as it goes into alternatives to pip, such as NeuroDebian, and mentions how to get pip and that we recommend using virtual environments. I would suggest that we remove this section:

nibabel/doc/source/index.rst

Lines 136 to 140 in b8b57cc

	Download and Installation
	=========================
	Please find detailed :ref:`download and installation instructions
	<installation>` in the manual.

And add that line to the long description/README. (We'll need to add a link in the README.rst header so <installation> resolves to the docs.

I believe the point of the license reprise is so that it could have links that render correctly within the docs. If we do the same and add rendering links to the header of README, we could remove the second. Similarly, I think just replacing the mailing list section in the README with the section from index.rst would be good.