Add support for labels and references for sections

[OlgaRedozubova]

This has not been implemented yet:

https://ru.overleaf.com/learn/latex/Cross_referencing_sections%2C_equations_and_floats

The label is set after the \section statement, i.e. the \label command should be added after the counter number for the section has been generated.

mmd:

\section{Introduction} \label{introduction}
This is an introductory paragraph with some dummy text. This section will be referenced later.

Link to section \ref{introduction}