Use Sphinx footnotes for ``Footnote``

### 📝 Description of the feature

Footnotes are not properly handled as the ``Footnote`` class is not modified from the ``Element`` class. Implementing Sphinx footnotes could be a good approach.
``XRef`` could also make use of Sphinx footnotes?

[Sphinx documentation](https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#footnotes) for reference

This was first raised by https://github.com/ansys/pymapdl/pull/3892#discussion_r2075144834

### 💡 Steps for implementing the feature

Add ``to_rst`` method in ``Footnote`` class.
Modify ``to_rst`` method in ``XRef`` class.

### 🔗 Useful links and references

``Footnote`` use cases:
- ``cnvtol``
- ``esol``: ``SOL - General Result Item and Component Labels`` - see https://github.com/ansys/pymapdl/pull/3892#discussion_r2075122923
- ``etable``
- ...

``XRef`` use cases:
- ``nsol`` in **NSOL - Valid Item and Component Labels** table
- ``rforce`` in **RFORCE - Valid Item and Component Labels**
- ...

Locate the Footnote and XRef classes and their existing to_rst methods, then review the linked Sphinx footnotes documentation and the cnvtol, esol, etable, nsol, and rforce use cases. Done means Footnote output follows Sphinx footnote syntax and XRef output handles the referenced cases correctly.

Written by the indexing model from the issue text.