Anthropic 1P/00_Tutorial risks insecure API key usage

## Issue

Notebook `Anthropic 1P/00_Tutorial_How-To.ipynb` asks users to enter their Anthropic API Key in a notebook cell.

This pattern comes with elevated risk that inexperienced users will accidentally commit their API key to a public repo.

## Suggested change

Here is one way to encourage a safe pattern for managing API keys:

1. In the `README`, instruct users to place their API key in a `.env` file.
2. Modify each notebook to use `python-dotenv` to load the environment variables each time the notebook runs.

## Details

1. Add `.env` to `.gitignore` to ensure it isn't committed
2. Add a `.env_template` file to the repo root which users can copy into a `.env` file. This can also include MODEL_NAME:
```bash
ANTHROPIC_API_KEY=
MODEL_NAME=claude-3-haiku-20240307
```

4. Add `!pip install python-dotenv` to all notebooks in `Anthropic 1P/`
5. Add the following env setup code to the import section of all notebooks in `Anthropic 1P/`

```python
import os
from dotenv import load_dotenv

load_dotenv();

API_KEY = os.getenv("ANTHROPIC_API_KEY")
MODEL_NAME = os.getenv("MODEL_NAME")
```

6. Update `README.md` to instruct users to create `.env` and add their API Key, e.g.

Copy `.env_template` to `.env`:

```bash
cp .env_template .env
```

Within `.env`, replace `` with your Anthropic API key (no quotes or brackets are needed).

> **Note:** `.env` is included in the `.gitignore` to avoid accidentally committing your API key. The notebooks in tutorial use the `python-dotenv` library to load environment variables from `.env` in each notebook. Be sure to avoid printing and committing the actual API key inside your notebooks.