feat(gitignore): add docs for command to merge and synchronize .gitignore files

Signed-off-by: Felipe Sayão Lobato Abreu <github@mentordosnerds.com>

Author: coisa

README.md

@@ -58,6 +58,9 @@ composer dev-tools wiki
 # Generate documentation frontpage and related reports
 composer dev-tools reports
 
+# Merges and synchronizes .gitignore files
+composer dev-tools gitignore
+
 # Installs and synchronizes dev-tools scripts, GitHub Actions workflows, .editorconfig, and ensures the repository wiki is present as a git submodule in .github/wiki
 composer dev-tools:sync
 ```

docs/api/commands.rst

@@ -42,3 +42,6 @@ resolution, configuration fallback, PSR-4 lookup, and child-command dispatch.
    * - ``FastForward\DevTools\Command\SyncCommand``
      - ``dev-tools:sync``
      - Synchronizes consumer-facing scripts and automation assets.
+   * - ``FastForward\DevTools\Command\GitIgnoreCommand``
+     - ``gitignore``
+     - Merges and synchronizes .gitignore files.

docs/running/specialized-commands.rst

@@ -138,3 +138,24 @@ Important details:
 - it copies missing workflow stubs, ``.editorconfig``, and ``dependabot.yml``;
 - it creates ``.github/wiki`` as a git submodule when the directory is
   missing.
+- it calls ``gitignore`` to merge the canonical .gitignore with the project's .gitignore.
+
+``gitignore``
+-------------
+
+Merges and synchronizes .gitignore files.
+
+.. code-block:: bash
+
+   composer dev-tools gitignore
+   composer dev-tools gitignore -- --source=/path/to/source/.gitignore --target=/path/to/target/.gitignore
+
+Important details:
+
+- it reads the canonical .gitignore from dev-tools and merges with the
+  project's existing .gitignore;
+- by default, the source is the packaged .gitignore and the target is the
+  project's root .gitignore;
+- duplicates are removed and entries are sorted alphabetically;
+- it uses the Reader, Merger, and Writer components from the GitIgnore
+  namespace.