Add documentation for the GoogleDrive source. (#171)

#108

Author: badmonster0

docs/docs/ops/sources.md

@@ -9,6 +9,8 @@ description: CocoIndex Built-in Sources
 
 The `LocalFile` source imports files from a local file system.
 
+### Spec
+
 The spec takes the following fields:
 *   `path` (type: `str`, required): full path of the root directory to import files from
 *   `binary` (type: `bool`, optional): whether reading files as binary (instead of text)
@@ -24,7 +26,45 @@ The spec takes the following fields:
 
     :::
 
+### Schema
 
 The output is a table with the following sub fields:
 *   `filename` (key, type: `str`): the filename of the file, including the path, relative to the root directory, e.g. `"dir1/file1.md"`
 *   `content` (type: `str` if `binary` is `False`, otherwise `bytes`): the content of the file
+
+## GoogleDrive
+
+The `GoogleDrive` source imports files from Google Drive.
+
+### Setup for Google Drive
+
+To access files in Google Drive, the `GoogleDrive` source will need to authenticate by service accounts.
+
+1.  Register / login in **Google Cloud**.
+2.  In [**Google Cloud Console**](https://console.cloud.google.com/), search for *Service Accounts*, to enter the *IAM & Admin / Service Accounts* page.
+    -   **Create a new service account**: Click *+ Create Service Account*. Follow the instructions to finish service account creation.
+    -   **Add a key and download the credential**: Under "Actions" for this new service account, click *Manage keys* → *Add key* → *Create new key* → *JSON*.
+      Download the key file to a safe place.
+3.  In **Google Cloud Console**, search for *Google Drive API*. Enable this API.
+4.  In **Google Drive**, share the folders containing files that need to be imported through your source with the service account's email address.
+    **Viewer permission** is sufficient.
+    -   The email address can be found under the *IAM & Admin / Service Accounts* page (in Step 2), in the format of `{service-account-id}@{gcp-project-id}.iam.gserviceaccount.com`.
+    -   Copy the folder ID. Folder ID can be found from the last part of the folder's URL, e.g. `https://drive.google.com/drive/u/0/folders/{folder-id}` or `https://drive.google.com/drive/folders/{folder-id}?usp=drive_link`.
+
+
+### Spec
+
+The spec takes the following fields:
+
+*   `service_account_credential_path` (type: `str`, required): full path to the service account credential file in JSON format.
+*   `root_folder_ids` (type: `list[str]`, required): a list of Google Drive folder IDs to import files from.
+*   `binary` (type: `bool`, optional): whether reading files as binary (instead of text).
+
+### Schema
+
+The output is a table with the following sub fields:
+
+*   `file_id` (key, type: `str`): the ID of the file in Google Drive.
+*   `filename` (type: `str`): the filename of the file, without the path, e.g. `"file1.md"`
+*   `mime_type` (type: `str`): the MIME type of the file.
+*   `content` (type: `str` if `binary` is `False`, otherwise `bytes`): the content of the file.