Write token docs (#2244) (#27)

Co-authored-by: Adrian Garcia Badaracco <1755071+adriangb@users.noreply.github.com>

docs/guides/advanced/05_creating_write_tokens.md

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-!!! warning "🚧 Work in Progress 🚧"
-    This page is yet to be written, [contact us](../../help.md) if you have any questions.
+To send data to **Logfire**, you need to create a write token.
+A write token is a unique identifier that allows you to send data to a specific **Logfire** project.
+If you set up Logfire according to the [first steps guide](../first_steps/index.md), you already have a write token locally tied to the project you created.
+But if you want to configure other computers to write to that project, for example in a deployed application, you need to create a new write token.
+
+You can create a write token by following these steps:
+
+1. Open the **Logfire** web interface at [logfire.pydantic.dev](https://logfire.pydantic.dev).
+2. Select your project from the **Projects** section on the left hand side of the page.
+3. Click on the ⚙️ **Settings** tab on the top right corner of the page.
+4. Select the **{} Write tokens** tab on the left hand menu.
+5. Click on the **Create write token** button.
+
+After creating the write token, you'll see a dialog with the token value.
+**Copy this value and store it securely, it will not be shown again**.
+
+Now you can use this write token to send data to your **Logfire** project from any computer or application.
+
+We recommend you inject your write token via environment variables in your deployed application.
+Set the token as the value for the environment variable `LOGFIRE_TOKEN` and logfire will automatically use it to send data to your project.
+
+## Setting `send_to_logfire='if-token-present'`
+
+You may want to not send data to logfire during local development, but still have the option to send it in production without changing your code.
+To do this we provide the parameter `send_to_logfire='if-token-present'` in the `logfire.configure()` function.
+If you set it to `'if-token-present'`, logfire will only send data to logfire if a write token is present in the environment variable `LOGFIRE_TOKEN` or there is a token saved locally.
+If you run tests in CI no data will be sent.
+
+You can also set the environmnet variable `LOGFIRE_SEND_TO_LOGFIRE` to configure this option.
+For example, you can set it to `LOGFIRE_SEND_TO_LOGFIRE=true` in your deployed application and `LOGFIRE_SEND_TO_LOGFIRE=false` in your tests setup.