DOC use pydata sphinx theme

[glemaitre]

closes #797

• Reorganize the API references page
• Rename the navbar with shorter name
• Move some examples
• Improve the index page with bootstrap card

[pep8speaks]

Hello @glemaitre! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/conf.py:

Line 23:1: E402 module level import not at top of file
Line 63:1: E402 module level import not at top of file

• In the file setup.py:

Line 23:11: F821 undefined name 'version'

Comment last updated at 2021-02-15 13:51:29 UTC

[codecov]

Codecov Report

Merging #801 (07e327f) into master (ce4e1f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #801   +/-   ##
=======================================
  Coverage   98.62%   98.62%           
=======================================
  Files          89       89           
  Lines        5881     5881           
  Branches      494      494           
=======================================
  Hits         5800     5800           
  Misses         80       80           
  Partials        1        1           

Impacted Files	Coverage Δ
imblearn/base.py	98.52% <ø> (ø)
imblearn/datasets/_imbalance.py	88.23% <ø> (ø)
imblearn/metrics/pairwise.py	100.00% <ø> (ø)

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