Serve does not inherit virtual enviroments

[gastmaier]

When using adoc serve, a new shell is created that does not inherit the virtual enviroment, if any, this causes adoc serve to fail with missing dependencies while the regular make html still works, making extra confusing for the user.

The current behaviour is

regular install inside the venv:
user:  works
dev: exit gracefully

development install inside the venv:
user: works
dev: works

development install outside the venv with packages inside the venv:
user: doesn't work
dev: doesn't work
* cannot access packages inside the venv, even the shell has it activated.

User refers to without the -r flag and dev with it.

References:
https://docs.python.org/3/library/sys.html#sys.executable
https://stackoverflow.com/questions/64761870/python-subprocess-doesnt-inherit-virtual-environment

[gastmaier]

Won't fix.
The user that wants to use the virtual environment should also do the development install inside the venv.

Future commits will improve warnings and add notes to clarify on the documentation.