Rollup merge of #85428 - leebradley:add-xpy-presetup-instructions, r=Mark-Simulacrum

Add x.py pre-setup instructions

This change adds pre-setup instructions that outline how x.py requires
python to be setup and how to work around the problem of x.py failing to
locate python, especially now that Ubuntu 20.04's dropped default python
command is causing people to encounter this issue regularly.

See also: #71818