docs: use markdown instead of rst

[bednar]

Closes #634

Proposed Changes

Migrate README file from rst to markdown.

Updated docs:

• https://github.com/influxdata/influxdb-client-python/tree/readme-to-md?tab=readme-ov-file#influxdb-client-python
• https://influxdb-client--639.org.readthedocs.build/en/639/#

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7a5f655) 90.40% compared to head (fad3946) 90.40%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #639   +/-   ##
=======================================
  Coverage   90.40%   90.40%           
=======================================
  Files          39       39           
  Lines        3503     3503           
=======================================
  Hits         3167     3167           
  Misses        336      336           

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[karel-rehor]

It appears the document docs/usage.rst generates an HTML document with only section headers. However, this is the same as in the master branch. Aside from this the PR looks legit.