docs: add How to cite ActivityWatch page

src/citing.rst

@@ -0,0 +1,30 @@
+How to cite ActivityWatch
+=========================
+
+If you use ActivityWatch in your research, please cite it. This helps us track
+academic impact and keeps the project sustainable.
+
+The canonical reference is the Zenodo DOI:
+
+**DOI:** `10.5281/zenodo.4957165 <https://doi.org/10.5281/zenodo.4957165>`_
+
+Ready-to-paste BibTeX for your methods section:
+
+.. code-block:: bibtex
+
+    @software{activitywatch,
+      author       = {Bjäreholt, Erik and Bjäreholt, Johan},
+      title        = {ActivityWatch},
+      year         = {2024},
+      publisher    = {Zenodo},
+      version      = {0.13.2},
+      doi          = {10.5281/zenodo.4957165},
+      url          = {https://github.com/ActivityWatch/activitywatch}
+    }
+
+A machine-readable `CITATION.cff <https://github.com/ActivityWatch/activitywatch/blob/master/CITATION.cff>`_
+is also available in the main repository, and GitHub will offer it via the
+"Cite this repository" button.
+
+If you publish a paper that uses ActivityWatch, we would love to hear about it —
+please let us know on `the forum <https://forum.activitywatch.net/>`_.

src/index.rst

@@ -51,6 +51,7 @@ Table of contents
    :caption: Meta
 
    privacy
+   citing
    history
    changelog