Updating docs

[lepmik]

• Docstrings for group, dataset, raw, attributes.
• A getting started, file with more elaborate tutorial
• Index is reduced to a bare example

[lepmik]

Will fix doctest failures asap

[codecov[bot]]

Codecov Report

Merging #82 into dev will decrease coverage by 0.4%. The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev      #82      +/-   ##
==========================================
- Coverage   95.57%   95.16%   -0.41%     
==========================================
  Files          11       11              
  Lines        1604     1469     -135     
==========================================
- Hits         1533     1398     -135     
  Misses         71       71

Impacted Files	Coverage Δ
tests/test_file.py	96.27% <0%> (-1.13%)	:arrow_down:
tests/test_group.py	94.39% <0%> (-1.06%)	:arrow_down:

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