Use new theme for docs

[dnerini]

This PR introduces a new theme for the documentation (https://sphinx-book-theme.readthedocs.io/en/latest/).

[codecov]

Codecov Report

Merging #259 (858d558) into master (10540d2) will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
- Coverage   80.92%   80.92%   -0.01%     
==========================================
  Files         142      142              
  Lines       10760    10758       -2     
==========================================
- Hits         8708     8706       -2     
  Misses       2052     2052              

Flag	Coverage Δ
unit_tests	80.92% <83.33%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pysteps/cascade/bandpass_filters.py	92.22% <ø> (ø)
pysteps/cascade/decomposition.py	90.21% <ø> (ø)
pysteps/decorators.py	99.14% <ø> (ø)
pysteps/downscaling/interface.py	90.90% <ø> (ø)
pysteps/extrapolation/interface.py	87.09% <ø> (ø)
pysteps/feature/interface.py	93.33% <ø> (ø)
pysteps/io/archive.py	82.22% <ø> (ø)
pysteps/io/exporters.py	52.35% <ø> (ø)
pysteps/io/importers.py	71.73% <ø> (ø)
pysteps/io/interface.py	65.57% <ø> (ø)
... and 61 more

---

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[RubenImhoff]

Looks good to me, great work @dnerini!

[dnerini]

Link to new doc: https://pysteps.readthedocs.io/en/refresh-docs/

[RubenImhoff]

This looks really good in the new docs theme, @dnerini!