chore: update README and remove xsl for plurimath, fixes #85

[ronaldtse]

Metanorma PR checklist

• [ ] Breaking changes (list related PRs)
• [ ] Documentation update required (create task for this)
• [ ] External dependency introduced (documentation update need)
• [ ] Gem with native library introduced

[codecov[bot]]

Codecov Report

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

Comparison is base (799af29) 95.19% compared to head (7a2c02e) 95.18%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #86 +/- ## ========================================== - Coverage 95.19% 95.18% -0.01% ========================================== Files 7 7 Lines 541 540 -1 ========================================== - Hits 515 514 -1 Misses 26 26 ```

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