docs: add existing project quickstart

[Mohamedajab]

Adds a short quickstart for contributors joining an existing Poetry project. It explains installing from pyproject.toml and poetry.lock, running commands in the project environment, and updating dependencies through Poetry.

Resolves #8131

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="docs/basic-usage.md" line_range="36-37" />
<code_context>
+poetry run pytest
+```
+
+Alternatively, use [`poetry env activate`]({{< relref "managing-environments#activating-the-environment" >}})
+to print the command that activates the environment in your shell.
+
+To add or remove a dependency, use [`poetry add`]({{< relref "cli#add" >}}) or
</code_context>
<issue_to_address>
**nitpick:** The quickstart tells users to run `poetry env activate` but only explains that it prints an activation command; running it by itself leaves the current shell unactivated, so users who follow this alternative do not enter the project's environment.

**Triggers:** When a contributor chooses shell activation instead of `poetry run`.

**Suggested fix:** Show how to evaluate the output for the relevant shell, such as `eval "$(poetry env activate)"` for Bash/Zsh, or explicitly tell users to run the printed command.

````suggestion
Alternatively, in Bash or Zsh, evaluate the output of [`poetry env activate`]({{< relref "managing-environments#activating-the-environment" >}}):

```bash
eval "$(poetry env activate)"
```
````
</issue_to_address>

Sourcery assessment

Approved.

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[sourcery-ai]

Sourcery assessment

Approved.