Encode LAMMPS version as a string to use in Colvars doc URLs

[giacomofiorin]

For some engines we are now maintaining a version-specific build of the documentation, e.g. the following:

• https://colvars.github.io/namd-3.0/colvars-refman-namd.html
• https://colvars.github.io/gromacs-2025/colvars-refman-gromacs.html
  and including those URLs in the main doc of the respective engines.

However, we are currently not doing it for LAMMPS, because its release version string is not known when we submit an update to the LAMMPS repo.

Following conversations with @akohlmey, a good way to address this would be do modify doc/utils/sphinx-config/conf.py.in in the LAMMPS repo to format the LAMMPS release string into a string macro without spaces.