Use Sphinx footnotes for ``Footnote``

[clatapie]

📝 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 for reference

This was first raised by ansys/pymapdl#3892 (comment)

💡 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 feat: post26 submodule pymapdl#3892 (comment)
• etable
• ...

XRef use cases:

• nsol in NSOL - Valid Item and Component Labels table
• rforce in RFORCE - Valid Item and Component Labels
• ...