Add usage documentation for pybabel concat and merge commands

 * Includes .rst file with detailed use cases and practical examples for pybabel's concat and merge utilities, outlining common scenarios, options, and best practices for managing PO files.

Author: soft-suroleb

docs/concat_merge_usage.rst

@@ -0,0 +1,52 @@
+Usage scenarios
+---------------
+
+1. Merging Multiple PO Files (`concat`)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Usage:**  
+`pybabel concat [options] <po_files>`
+Suppose you manage a project with several PO files for the same language (for example, modules or plugins have their own translations), and you want to combine them into a single file for further work or for delivery to translators.
+
+**Example:**
+
+.. code-block:: shell
+
+    pybabel concat -o merged.po module1.po module2.po module3.po
+
+**Features:**
+
+- If the same string has different translations in different files, the resulting file for that string will include a special comment ``#-#-#-#-#  <file> (PROJECT VERSION)  #-#-#-#-#`` and the message will be marked with the ``fuzzy`` flag—this is useful for later manual conflict resolution.
+- You can keep only unique strings using the ``-u`` (`--less-than=2`) option.
+- Use `--use-first` to take only the first encountered translation for each string, skipping automatic merging of multiple options.
+- Output can be sorted alphabetically or by source file (options `-s`, `-F`).
+
+**Typical Use Case:**
+
+    A project has translations from different teams. Before releasing, you need to gather all translations into one file, resolve possible conflicts, and provide the finalized version to translators for review.
+
+
+2. Updating Translations with a Template and Compendium (`merge`)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Usage:**  
+`pybabel merge [options] def.po ref.pot`
+You need to update an existing translation file (`def.po`) based on a new template (`ref.pot`), reusing translations from an additional translation memory (compendium).
+
+**Example:**
+
+.. code-block:: shell
+
+    pybabel merge -C my-compendium.po --backup def.po ref.pot
+
+**Features:**
+
+- The compendium (`-C`) allows you to pull translations from a shared translation memory. Multiple compendiums can be used.
+- By default, translations from the compendium are used only for new or missing entries in `def.po`.
+- The `--compendium-overwrite` option allows overwriting existing translations with those found in the compendium (helpful for terminology standardization).
+- When a translation from the compendium is used, a comment is automatically added (this can be disabled with `--no-compendium-comment`).
+- The `--backup` flag saves a backup copy of your file before updating (`~` suffix by default, configurable with `--suffix`).
+
+**Typical Use Case:**
+
+    After a release, a new translation template is provided. The team decides to enrich the translation by leveraging a common compendium in order to improve quality and unify terms. The merge command is run with the compendium and backup options enabled.