[docs/installation.md] Document how to install to custom locations

[SamuelMarks]

Summary

Closes #5449

Test Plan

$ curl --proto '=https' --tlsv1.2 -OLsSf https://github.com/astral-sh/uv/releases/download/0.2.29/uv-installer.sh
$ env -i CARGO_HOME=/tmp/foo/cargo HOME=/tmp/foo/home /usr/bin/bash uv-installer.sh --no-modify-path
downloading uv 0.2.29 x86_64-unknown-linux-gnu
installing to /tmp/foo/cargo/bin
  uv
  uvx
everything's installed!
$ tree -a /tmp/foo
/tmp/foo
├── cargo
│   └── bin
│       ├── uv
│       └── uvx
└── home
    └── .config
        └── uv
            └── uv-receipt.json

6 directories, 3 files

[zanieb]

Sorry I was on vacation — I'll get to this soon!

[zanieb]

I'm a little worried this is too complicated. I wonder if we should add a custom environment variable to make this possible? Is there a reason the download is separate from the install invocation?

[zanieb]

This is documented in a simpler way at https://docs.astral.sh/uv/getting-started/installation/#configuring-installation

See #7107

Thanks for contributing though this was really helpful while we waited for upstream to add simpler support