Convert InformalTables

### 📝 Description of the feature

``InformalTables`` are ignored for the moment.

First mention in https://github.com/ansys/pymapdl/pull/3385#discussion_r1907348857.

### 💡 Steps for implementing the feature

``Table`` should be the parent class of ``InformalTable``.
This leads to needed changes with the ``to_rst`` methods as the ``fcache`` parameter needs to be added in some of them.

``InformalTable`` can look as follow which imply some changes in the ``THead.to_rst()`` method:
check the [CNVTOL](https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v242/en/ans_cmd/Hlp_C_CNVTOL.html?q=CNVTOL) documentation

### 🔗 Useful links and references

_No response_