Scrape correct download link for Doxygen.

The Windows version of the GitHub Actions tests require
downloading Doxygen binaries. Every now and then this
step would break when Doxygen updates their version numbers.
Added a Python script to scrape the Doxygen website to
determine the correct URL to download.
commit_hook.bat calls the Python script to determine
the URL, so I don't have to keep manually updating it.

generate/commit_hook.bat

@@ -1,4 +1,5 @@
 @echo off
+setlocal EnableDelayedExpansion
 REM --------------------------------------------------------------------------------
 REM     Astronomy Engine - GitHub Actions steps for Windows.
 REM     This batch file is executed on every push to GitHub.
@@ -11,7 +12,16 @@ echo.commit_hook: Repo root = %cd%
 echo.commit_hook: Installing Doxygen.
 md bin
 cd bin
-curl -o doxygen.zip https://www.doxygen.nl/files/doxygen-1.9.4.windows.x64.bin.zip || exit /b 1
+set DOXYGENURL=
+for /f %%x in ('py doxygen_download_link.py') do (
+    set DOXYGENURL=%%x
+)
+if not defined DOXYGENURL (
+    echo.commit_hook: FAIL: could not determine URL to download Doxygen binaries for Windows.
+    exit /b 1
+)
+echo.commit_hook: Downloading: !DOXYGENURL!
+curl -o doxygen.zip !DOXYGENURL! || exit /b 1
 7z x doxygen.zip || exit /b 1
 del doxygen.zip