[mlir][docgen] Display full attribute descriptions in expandable regions (#67009)

This updates the table of op attributes so that clicking the summary
expands to show the complete description.

```
   Attribute | MLIR Type | Description
   <name>      <type>      ▶️ <summary>  <-- Click to expand
```

Enum attributes have now also been updated to generate a description
that lists all the cases (with both their MLIR and C++ names). This
makes viewing enums on the MLIR docs much nicer.

**Example**

Default view:

![image](https://github.com/llvm/llvm-project/assets/11597044/922669c7-b838-4230-bcfd-a77cde0f335d)

Expanded:

![image](https://github.com/llvm/llvm-project/assets/11597044/41da086e-a5ce-45dd-9f44-9d10a4d5f2e1)

---

This requires: https://github.com/llvm/mlir-www/pull/158 (adds a very
simple markdown shortcode)