sagemathgh-37821: Fix category of ModularFormsRing and documentation …

…cleanup

    
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The goal of this PR is twofold:

First, we fix the category of modular forms ring as per PR sagemath#37797 to
make the following code work:
```
sage: M = ModularFormsRing(1)
sage: M['z', 'w']
Multivariate Polynomial Ring in z, w over Ring of Modular Forms for
Modular Group SL(2,Z) over Rational Field
```

Second, we do a documentation and PEP8 cleanup of the class
`ModularFormsRing`.

CC: @fchapoton

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- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#37821
Reported by: David Ayotte
Reviewer(s): Kwankyu Lee