Handle ``informaltable`` elements

### 📝 Description of the feature

At the moment, informal tables are not supported and point to the original documentation fur further information. This needs to be modified

### 💡 Steps for implementing the feature

A few places where ``informaltables`` elements can be found:
- ``dmat``
- ``fft``
- ``vec``
- ``prcint``: https://github.com/ansys/pymapdl/pull/4003#discussion_r2142332118

### 🔗 Useful links and references

_No response_

Start by locating the converter logic that handles table elements, then inspect the informal-table occurrences in dmat, fft, vec, and prcint. Compare their XML structure with currently supported tables and use the linked prcint discussion for context. Done means informal tables are converted instead of pointing to the original documentation.

Written by the indexing model from the issue text.