Improve UX when `uv add` or `uv sync` are run in a directory with no `pyproject.toml`

[edmorley]

Summary

Currently if uv add or uv sync are run in a directory with no pyproject.toml, the resultant error message is:

$ mkdir testcase-uv && cd $_
$ uv add urllib3
error: No `pyproject.toml` found in current directory or any parent directory
$ uv sync
error: No `pyproject.toml` found in current directory or any parent directory

To improve the UX for users migrating from pip (or uv's uv pip interface), it would help if this error message hinted that the user should probably run uv init or else see the uv getting started docs.

Example

Perhaps something roughly like this?

$ uv add urllib3
error: No `pyproject.toml` found in current directory or any parent directory.
To create one run `uv init`, or see: https://docs.astral.sh/uv/guides/projects/

[zanieb]

We probably want to use the hint: or help: formatting here. Makes sense though.

So far, we don't link to the documentation because it's not versioned and therefore not guaranteed to be stable.