update the github workflow document for recent changes

  * every new command or style should have documentation. The names of

  source files (c++ and manual) should follow the name of the style.

  (example: `src/fix_nve.cpp`, `src/fix_nve.h` for `fix nve` command,

  implementing the class `FixNVE`, documented in `doc/src/fix_nve.txt`)

  implementing the class `FixNVE`, documented in `doc/src/fix_nve.rst`)

  * all new style names should be lower case, the must be no dashes,

  blanks, or underscores separating words, only forward slashes.

  * 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`

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

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

  * if documentation is (still) provided as a .txt file, convert to .rst

  and remove the .txt file. For files in doc/txt the conversion is automatic.

  * remove all .txt files in `doc/txt` that are out of sync with their .rst counterparts in `doc/src`

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

  * check style tables and command lists with `make style_check`

  * 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

  * Code should follow the C++-98 standard. C++-11 is only accepted

  in individual special purpose packages

  * indentation is 2 spaces per level

  * there should be NO tabs and no trailing whitespace

  * there should be NO tabs and no trailing whitespace (review the "checkstyle" test on pull requests)

  * header files, especially of new styles, should not include any

  other headers, except the header with the base class or cstdio.

  Forward declarations should be used instead when possible.