Explicit documentation that rez-pip can only install packages with wheels

[brycegbrazen]

Currently the documentation doesn't explicitly mention that rez-pip can only install packages that have wheels. We should make sure to add this in the docs as a prerequisite or remove this requirement.

[brycegbrazen]

I do want to mention a specific example. With old rez-pip I could run rez-pip "docopt>=0.5" --install, and the conversion worked with the following output:

$ rez-pip "docopt>=0.5" --install
18:22:12 INFO     Trying to use pip from python package
18:22:13 INFO     Found pip-22.3.1 inside \\vm-fs-003\prod\sww\rez\packages\python\3.10.10\package.py. Will use it via c:\.rez\cache\python\3.10.10\001f\a\python\python.exe
18:22:13 INFO     Installing 'docopt>=0.5' with pip taken from 'c:\\.rez\\cache\\python\\3.10.10\\001f\\a\\python\\python.exe'
Looking in indexes: https://pypi.org/simple, http://vm-docker-003:8080/simple
Collecting docopt>=0.5
  Using cached docopt-0.6.2.tar.gz (25 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
Building wheels for collected packages: docopt
  Building wheel for docopt (pyproject.toml) ... done
  Created wheel for docopt: filename=docopt-0.6.2-py2.py3-none-any.whl size=13773 sha256=afbe1e33342a923cdf74d6c7b8a12e97d64137f4c9906de0fe543efb6d721261
  Stored in directory: c:\users\bryce.gattis\appdata\local\pip\cache\wheels\7c\d7\8d\2156234738063e3d4a39ba77dc677046100e62766b53807189
Successfully built docopt
Installing collected packages: docopt
Successfully installed docopt-0.6.2

Attempting the same install with rez-pip2 looks like this:

$ rez-pip2 "docopt>=0.5" --python-version 3.10
INFO     Installing requested packages for Python 3.10.10
ERROR: Could not find a version that satisfies the requirement docopt>=0.5 (from versions: none)
ERROR: No matching distribution found for docopt>=0.5
rez_pip.exceptions.PipError: Failed to run pip command: 'c:\\.rez\\cache\\python\\3.10.10\\001f\\a\\python\\python.exe c:\\program files\\rez\\lib\\site-packages\\rez_pip\\data\\pip.pyz install -q docopt>=0.5 --disable-pip-version-check --dry-run --ignore-installed --python-version=3.10.10 --only-binary=:all: --target=/tmp/asd --disable-pip-version-check --report C:\\Users\\BRYCE~1.GAT\\AppData\\Local\\Temp\\pip-install-output1vg8e4w3'

Pip reported this:

ERROR: Could not find a version that satisfies the requirement docopt>=0.5 (from versions: none)
ERROR: No matching distribution found for docopt>=0.5