[DOC] Add note about int eq labels in math directive

[najuzilu]

This PR adds a note directive with information that integer math labels cannot be referenced. This could be helpful to LaTeX users who are used to integer equation labels and references.

[choldgraf]

Does this only apply to labels that are only integers? Or labels that also include integers? (e.g. would eq1 work?)

[najuzilu]

Only labels that are integers or start with an integer (e.g. 1eq doesn't work either). The second follows from the first but it might not be intuitive for other users. If you think we should specify, I can update to this

Integer equation labels and labels which start with an integer cannot be referenced and
will throw a warning message if referenced. For example, :label: 1 and :label: 1eq cannot
be referenced.

[choldgraf]

Your suggested text there looks great 👍

[choldgraf]

Thanks!!