Docs: How to add Bitbucket as data source. (#14846)

### What problem does this PR solve?

Added a guide on integrating Bitbucket as an external data source.

### Type of change

- [x] Documentation Update

docs/guides/dataset/add_data_source/add_bitbucket.md

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+---
+sidebar_position: 16
+slug: /add_confluence
+sidebar_custom_props: {
+  categoryIcon: SiGoogledrive
+}
+---
+
+# Add Bitbucket
+
+Integrate Bitbucket as a data source.
+
+---
+
+This guide outlines the integration of Bitbucket as a data source for RAGFlow.
+
+## Prerequisites
+
+Before starting, ensure you have the following:
+
+- **Bitbucket API token:** A Personal Access Token (PAT) with the appropriate scopes or permissions.
+- **Repository URL:** The full URL of the repository you wish to index.
+- **Workspace ID:** The unique identifier for your Bitbucket workspace.
+
+## Configuration steps
+
+### Define Bitbucket as an external data source
+
+Navigate to the **Connectors** or **External Data Source** section in the RAGFlow Admin Panel and select **Bitbucket**. Fill in the connector details in the popup window:
+
+- **Name**: A descriptive name for this connector.
+- **Bitbucket Account Email**: The email address for your Bitbucket account.
+- **Bitbucket API Token**: The API token with proper permissions created in the previous step.
+- **Workspace** The `WORKSPACE_NAME` from your Bitbucket URL, e.g., `https://bitbucket.org/{WORKSPACE_NAME}/...`
+- **Index Mode**
+  - **Workspace**: (Default) Indexes all repositories in the workspace.
+  - **Repositories**: Indexes specified repositories in the workspace.
+    - **Repository Slugs**: A comma-separated list of repository slugs, e.g., `repo2,repo2`.
+  - **Projects**: Indexes specified projects in the workspace.
+    - **Projects**: A comma-separated list of project keys, e.g., `PROJ1,PROJ2`.
+
+*RAGFlow validates the connection immediately and indexes all pull requests from the specified repos or projects.*
+
+### Link to a dataset
+
+Credentials alone do not trigger indexing. You must link the data source to a specific dataset:
+
+1. Navigate to the **Dataset** tab.
+2. Select or create the target Dataset.
+3. Navigate to the Dataset's **Configuration** page and select **Link data source**.
+4. Choose the previously created Bitbucket connector in the popup window.