DOC: Added nibabel's svg logo

Colors change from nipy svg logo to nibabel svg logo:
1. lemonchiffon: #fdfdcb -> tan: #cec890
2. olive: #649700 -> steelblue: #2f83c8
3. darkolivegreen: #49660d -> black: #0c0f05

1 and 2 according to:
https://github.com/nipy/nibabel/blob/c0ca2f87b7d7ec85715540fbb126d8970876b46e/doc/source/_static/nibabel.css

Source svg:
https://nipy.org/img/nipy.svg

Author: jond01