Add CITATION.cff for automatic academic citation (#1351)

* Add CITATION.cff for automatic academic citation

A `CITATION.cff` file has been added to the project to facilitate the generation of accurate academic citations directly from the GitHub page. This allows users to easily copy the citation and paste it into their academic papers. The file includes the project's information like title, authors, repository code, url, keywords, and license.

* Fixed category name

CHANGES/1351.doc.rst

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+Added `CITATION.cff` file for automatic academic citation generation.
+Now you can copy citation from the GitHub page and paste it into your paper.

CITATION.cff

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+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: aiogram
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Oleksandr
+    family-names: O
+    email: im@aiogram.dev
+repository-code: 'https://github.com/aiogram/aiogram'
+url: 'https://aiogram.dev'
+abstract: >-
+  aiogram is a modern and fully asynchronous framework for
+  Telegram Bot API written in Python using asyncio
+keywords:
+  - aiogram
+  - telegram
+  - bot
+license: MIT