doc(ingestion/gc): Add doc for GC source (#12296)

metadata-ingestion/docs/sources/datahubgc/README.md

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+# DataHub Garbage Collection Source Documentation
+
+## Overview
+
+The DataHub Garbage Collection (GC) source is a maintenance component responsible for cleaning up various types of metadata to maintain system performance and data quality. It performs multiple cleanup tasks, each focusing on different aspects of DataHub's metadata.
+
+## Cleanup Tasks
+
+### 1. Index Cleanup
+
+Manages Elasticsearch indices in DataHub, particularly focusing on time-series data.
+
+#### Configuration
+```yaml
+source:
+  type: datahub-gc
+  config:
+      truncate_indices: true
+      truncate_index_older_than_days: 30
+      truncation_watch_until: 10000
+      truncation_sleep_between_seconds: 30
+```
+
+#### Features
+- Truncates old Elasticsearch indices for the following timeseries aspects:
+    - DatasetOperations
+    - DatasetUsageStatistics
+    - ChartUsageStatistics
+    - DashboardUsageStatistics
+    - QueryUsageStatistics
+    - Timeseries Aspects
+- Monitors truncation progress
+- Implements safe deletion with monitoring thresholds
+- Supports gradual truncation with sleep intervals
+
+### 2. Expired Token Cleanup
+
+Manages access tokens in DataHub to maintain security and prevent token accumulation.
+
+#### Configuration
+```yaml
+source:
+  type: datahub-gc
+  config:
+    cleanup_expired_tokens: true
+```
+
+#### Features
+- Automatically identifies and revokes expired access tokens
+- Processes tokens in batches for efficiency
+- Maintains system security by removing outdated credentials
+- Reports number of tokens revoked
+- Uses GraphQL API for token management
+
+### 3. Data Process Cleanup
+
+Manages the lifecycle of data processes, jobs, and their instances (DPIs) within DataHub.
+
+#### Features
+- Cleans up Data Process Instances (DPIs) based on age and count
+- Can remove empty DataJobs and DataFlows
+- Supports both soft and hard deletion
+- Uses parallel processing for efficient cleanup
+- Maintains configurable retention policies
+
+#### Configuration
+```yaml
+source:
+  type: datahub-gc
+  config:
+    dataprocess_cleanup:
+      enabled: true
+      retention_days: 10
+      keep_last_n: 5
+      delete_empty_data_jobs: false
+      delete_empty_data_flows: false
+      hard_delete_entities: false
+      batch_size: 500
+      max_workers: 10
+      delay: 0.25
+```
+
+### Limitations
+
+- Maximum 9000 DPIs per job for performance
+
+
+### 4. Execution Request Cleanup
+
+Manages DataHub execution request records to prevent accumulation of historical execution data.
+
+#### Features
+- Maintains execution history per ingestion source
+- Preserves minimum number of recent requests
+- Removes old requests beyond retention period
+- Special handling for running/pending requests
+- Automatic cleanup of corrupted records
+
+#### Configuration
+```yaml
+source:
+  type: datahub-gc
+  config:
+    execution_request_cleanup:
+      enabled: true
+      keep_history_min_count: 10
+      keep_history_max_count: 1000
+      keep_history_max_days: 30
+      batch_read_size: 100
+      runtime_limit_seconds: 3600
+      max_read_errors: 10
+```
+
+### 5. Soft-Deleted Entities Cleanup
+
+Manages the permanent removal of soft-deleted entities after a retention period.
+
+#### Features
+- Permanently removes soft-deleted entities after retention period
+- Handles entity references cleanup
+- Special handling for query entities
+- Supports filtering by entity type, platform, or environment
+- Concurrent processing with safety limits
+
+#### Configuration
+```yaml
+source:
+  type: datahub-gc
+  config:
+    soft_deleted_entities_cleanup:
+      enabled: true
+      retention_days: 10
+      batch_size: 500
+      max_workers: 10
+      delay: 0.25
+      entity_types: null  # Optional list of entity types to clean
+      platform: null      # Optional platform filter
+      env: null          # Optional environment filter
+      query: null        # Optional custom query filter
+      limit_entities_delete: 25000
+      futures_max_at_time: 1000
+      runtime_limit_seconds: 7200
+```
+
+### Performance Considerations
+- Concurrent processing using thread pools
+- Configurable batch sizes for optimal performance
+- Rate limiting through configurable delays
+- Maximum limits on concurrent operations
+
+## Reporting
+
+Each cleanup task maintains detailed reports including:
+- Number of entities processed
+- Number of entities removed
+- Errors encountered
+- Sample of affected entities
+- Runtime statistics
+- Task-specific metrics