Fix shell-commands.rst

[FlanOfHarmony]

Hi. First off: nice job on platformIO! It's a great project (Especially when there's no arduino package anymore on fedora). I just hope that one day you'll be able to get rid of microsoft's evil VScode Cpp extension as discussed here

But anyway, here's a commit, which deals with two issues I have with the instructions in shell-commands.rst.

1. The first method is causing issues with python. Specifically, adding the $HOME/.platformio/penv/bin (wich contains a python3 symlink) breaks my python install. Without adding this directory to my $PATH variable, I'm having some ModuleNotFoundError errors while executing some (non related to PlatformIO) python programs i.e. something's wrong with python's virtual environment. I'm not a big python virtual environment expert so I don't know why, but without the directory in the $PATH variable everything is fine. To fix this I propose to use symlinks and ~/.local/bin instead (which is usually already filled with python files).

2. On my freshly installed system, there is no /usr/share/bin/ directory. According to a quick google search, this is uncommon but can happen. Also according to the warning on this page of PlatformIO's documentation, requiring administrator permissions is against the spirit of PlatformIO so I thought about removing the method entirely, but I guess there are some use cases to a system-wide symlinks? When not "logged in as the user PlatformIO is installed and configured for"? Idk but I left it and added a small mkdir -p in case others do not have this directory on their systems.

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[ivankravets]

Thanks for the PR!