📝(backend) add docs

i am documenting the index command

Signed-off-by: charles <charles.englebert@protonmail.com>

Author: mascarpon3

docs/commands/index.md

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+# Index Command
+
+The `index` management command is used to index documents to the remote search indexer.
+
+It sends an asynchronous task to the Celery worker.
+
+## Usage
+
+### Command line
+
+```bash
+python manage.py index \
+  --lower-time-bound "2024-01-01T00:00:00" \
+  --upper-time-bound "2024-01-31T23:59:59" \
+  --batch-size 200 \
+  --crash-safe-mode
+```
+
+### Django Admin
+
+The command is available in the Django admin interface:
+
+1. Go to `/admin/`
+2. Click on **"Run Indexing"** in the CORE section
+3. Fill in the form with the desired parameters
+4. Click **"Run Indexing Command"**
+
+### Make Command
+
+```bash
+# Basic usage with defaults
+make index
+
+# With custom parameters
+make index batch_size=200 lower_time_bound="2024-01-01T00:00:00" crash_safe_mode=true
+
+# All parameters
+make index batch_size=200 \
+  lower_time_bound="2024-01-01T00:00:00" \
+  upper_time_bound="2024-01-31T23:59:59" \
+  crash_safe_mode=true
+```
+
+## Parameters
+
+### `--batch-size`
+- **type:** Integer
+- **default:** `50`
+- **description:** Number of documents to process per batch. Higher values may improve performance but use more memory.
+
+### `--lower-time-bound`
+- **optional**: true
+- **type:** ISO 8601 datetime string
+- **default:** `None`
+- **description:** Only documents updated after this date will be indexed.
+
+### `--upper-time-bound`
+- **optional**: true
+- **type:** ISO 8601 datetime string
+- **default:** `None`
+- **description:** Only documents updated before this date will be indexed.
+
+### `--crash-safe-mode`
+- **type:** Boolean flag
+- **default:** `False`
+- **description:** When enabled, the command orders documents by `updated_at` and stores the last indexed document's timestamp in cache. This allows resuming indexing from the last successful batch in case of a crash. However, it is more computationally expensive due to sorting.
+
+