This hopefully helps a bit when upgrading CMake code that uses
`exec_program()` command.

Changes:
- Used third-person phrasing for the command description.
- The `exec_program()` command arguments in signature updated a bit.
- Mention of CMakeLists.txt file removed as this command also worked in
  CMake scripts and other modes.
- Added examples section.

Most of CMake commands use it. Fix a few of them to do the same.

Use inline reStructuredText markup and add cross-references in more
places.

Run the convert-help.bash script to convert documentation:

 ./convert-help.bash "/path/to/CMake-build/bin"

Then remove it.