Fix error in parallelization scheme

[marinegor]

Changes made in this Pull Request:

• fix figure as mentioned in #4729 discussion
• updates docs accordingly
• additionally mentions progress about parallelization implementation in docs

PR Checklist

• [ ] Tests?
• [x] Docs?
• [ ] CHANGELOG updated?
• [ ] Issue raised/referenced?

Developers certificate of origin

• [ ] I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4760.org.readthedocs.build/en/4760/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.63%. Comparing base (d2729f7) to head (ca9c452). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4760 +/- ## =========================================== - Coverage 93.65% 93.63% -0.03% =========================================== Files 175 187 +12 Lines 21565 22640 +1075 Branches 3023 3024 +1 =========================================== + Hits 20196 21198 +1002 - Misses 925 998 +73 Partials 444 444 ```

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[orbeckst]

@IAlibay I assigned the PR to you so you can make the decision about merging despite 2.8.0 feature freeze.