update workflow tips for checking new LAMMPS contributions

  * new style docs should be added to the "overview" files in

  `doc/src/Commands_*.txt`, `doc/src/{fixes,computes,pairs,bonds,...}.txt`

  and `doc/src/lammps.book`

  * new files in packages should be added to `src/.gitignore`

  * check whether manual cleanly translates with `make html` and `make pdf`

  * check spelling of manual with `make spelling` in doc folder

  * new source files in packages should be added to `src/.gitignore`

  * removed or renamed files in packages should be added to `src/Purge.list`

  * C++ source files should use C++ style include files for accessing

  C-library APIs, e.g. `#include <cstdlib>` instead of `#include <stdlib.h>`.