docs: add pipx/uv install instructions and externally-managed-environment troubleshooting

[xamelllion]

Problem

The Installation section of docs/README.md only suggests pip install kaggle.
On many modern systems this fails with:

error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.
    ...
note: If you believe this is a mistake, please contact your Python installation
or OS distribution provider. You can override this, at the risk of breaking your
Python installation or OS, by passing --break-system-packages.

This is the expected behavior on distributions that follow
PEP 668 (e.g. Debian 12+, Ubuntu 23.04+,
Fedora, Arch, Homebrew Python). I ran into it on [OS, e.g. Ubuntu 24.04] with Python 3.12.

The docs currently don't mention this, so users following the instructions
literally get stuck.

Changes

Only docs/README.md is changed (Installation section):

• Recommend pipx install kaggle / uv tool install kaggle as the primary
  method: it works on all platforms and doesn't require users to manage
  virtual environments.
• Keep plain pip install kaggle as an alternative.
• Add a Troubleshooting note for externally-managed-environment, pointing
  to pipx/uv first and mentioning --break-system-packages as a last resort,
  with a warning about the risks.
• Move the existing Command kaggle not found note into the same Troubleshooting block.

No code changes.

Notes

• The root README.md links to this page for "Additional installation instructions",
  so I left it unchanged. Happy to update it too if you'd prefer.

[stevemessick]

@xamelllion What does this mean? issue creation is restricted

You have to have a GitHub account to create an issue, but otherwise I'm not aware of any restrictions.

Your PR looks good so far. I'll finish my review soon.

[stevemessick]

/gcbrun

[xamelllion]

@stevemessick

My bad. Looks like my agent saw that message while browsing the issues page without being logged in.
Updated description by myself.

[stevemessick]

@xamelllion All good, thanks for the update!

[stevemessick]

@rosbo Just wanted to check if you have any concerns about changing the recommended installation procedure.