Update contributing, testing and add documentation intro.

[benedekrozemberczki]

Summary

Minor changes in the readme, contribution and documentation.

• [ ] Unit tests provided for these changes
• [ ] Documentation and docstrings added for these changes using the sphinx style

Changes

• Update test running in the README
• Update contributing for the library
• Add Arxiv badge to the library
• Add docs intro
• Docs structure updated
• Citation in docs

[codecov-commenter]

Codecov Report

Merging #88 (7d1abce) into main (20f0e85) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 7d1abce differs from pull request most recent head 8ec8588. Consider uploading reports for the commit 8ec8588 to get more accurate results

@@           Coverage Diff           @@
##             main      #88   +/-   ##
=======================================
  Coverage   94.65%   94.65%           
=======================================
  Files          34       34           
  Lines        1478     1478           
=======================================
  Hits         1399     1399           
  Misses         79       79           

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