Install guide: `make sync` fails without the three sibling repos

[tamohannes]

Following docs/how-to/install.md exactly on a clean machine fails at the first step.

Repro

git clone https://github.com/nustackdev/nu
cd nu
make sync

Using CPython 3.12.13
Creating virtual environment at: .venv
error: Distribution not found at: file:///Users/me/code/virtuals
make: *** [sync] Error 2

Cause

pyproject.toml declares three editable path dependencies pointing one level up:

[tool.uv.sources]
virtuals-py   = { path = "../virtuals", editable = true }
invisibles-py = { path = "../invisibles", editable = true }
kh57          = { path = "../kh57", editable = true }

The install page never mentions them, so a first time user hits a bare "Distribution not found" with no indication that three more clones are required, or that they must be siblings of nu/ rather than nested inside it.

Suggested patch

In docs/content/docs/how-to/install.md, under "Install the Python side":

Nu resolves `virtuals`, `invisibles`, and `kh57` as editable path dependencies one
directory up, so clone all four as siblings:

```bash
mkdir -p ~/code && cd ~/code
git clone https://github.com/nustackdev/nu
git clone https://github.com/nustackdev/virtuals
git clone https://github.com/nustackdev/invisibles
git clone https://github.com/nustackdev/kh57
cd nu
make sync
```

Adding it to "Common errors" as well would catch anyone who cloned into a nested layout.

Happy to send this as a PR if forking gets enabled; the repo currently has allow_forking: false and read only collaborator access.

[tamohannes]

Fix is written and tested locally on a branch (fix/install-docs-and-version), covering this and #3 in one commit:

• docs/content/docs/how-to/install.md: the three sibling clones in the install steps, plus a Common errors entry for the nested layout case.
• src/nu/__init__.py: defines __version__ via importlib.metadata, with a PackageNotFoundError fallback for source trees with no dist-info. import nu; nu.__version__ now returns 0.1.0, so the documented verify step works as written.

I cannot open a PR: this account has read only access (push: false) and allow_forking is off, so there is no fork to push from either. Two ways forward, whichever suits you:

1. Grant push access and I will open the PR directly.
2. Enable forking and I will open it cross fork.
3. Or say the word and I will paste the full diff here for you to apply.

Found while porting an app onto Nu, so both of these are real first run experience rather than a code read.