docs: Improve Linux cmake getting started documentation (IDFGH-1020)

[Roosted7]

Whilst I was going through the new cmake getting started guide, I think I encountered two minor imperfections in the documentation.

Python2 usage

Firstly, if the default python command defaults to version 3 instead of 2, the guide recommends to run idf.py with the older python2 interpreter. But the the way it was suggested will not work, since the idf.py script can not be passed as an argument to the python2 interpreter but its path must then be used.
In this commit, I used a nested which command to find the path.

Discrepancy pip usage

In step 4 of the getting the started, pip is used to install the requirements.txt file. In the Ubuntu/Debian prerequisite package list pip is included, but in the Arch version it is not. It seems to make sense to add it to the Arch package list as well. (Even though imho it would make more sense to remove step 4 and do not use pip to install the packages again for a specific user)

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