Fix requirements.txt & remove Raspberry manual setup

[jredrejo]

Removing Raspberry Pi manual instructions because:

• Current Raspberry Pi manual setup is not valid anymore after v0.15 release
• Even when it was valid, it's a complex setup that requires important skills and it was a source of problems in the support forums

Also fixed Python requirements.txt to pin sphinx_rtd_theme because newer versions are incompatible with the current build setup

Closes: #235