doc: Use an environment variable less likely to vary between builds SHELL is less user-specific than USER, and can easily be forced to a known value for reproducible builds.
doc: Use an example shell command whose output is less likely to vary
The path returned by $(which grep) might change depending on whether the system is merged-/usr or whether it has a local /usr/local/bin/grep, and the result of running ls can depend on factors such as the time zone.
Using a different command here also helps to illustrate that shell syntax such as "&&" is accepted here, and using "command -v" avoids deprecation warnings from debianutils which(1).
doc: Use python3 instead of python in examples This makes the examples more illustrative, by running a command that actually exists. It also avoids embedding the full path to the build directory in the generated documentation, which is bad for reproducible builds.
