➕ Add pip-tools dependency

The direct dependencies ("top level packages")
must now be listed in `requirements.in.

We should now use `pip-compile` to create `requirements.txt`.
    pip-compile

` pip-compile` reads `requirements.in` to generate `requirements.txt`
with the pinned direct and indirect dependencies.

The `pip-compile` program is part of the `pip-tools` package.
We need to install it first:
    python -m pip install pip-tools

IMPORTANT:
Do not to add packages/dependencies directly into `requirements.txt`
but in `requirements.in`.

requirements.in

@@ -0,0 +1,4 @@
+sphinx==5.3.0
+sphinx-copybutton==0.5.2
+sphinx-rtd-theme==1.2.0
+myst-parser==0.18.1

requirements.txt

@@ -1,4 +1,78 @@
+#
+# This file is autogenerated by pip-compile with Python 3.12
+# by the following command:
+#
+#    pip-compile
+#
+alabaster==0.7.16
+    # via sphinx
+babel==2.16.0
+    # via sphinx
+certifi==2024.8.30
+    # via requests
+charset-normalizer==3.3.2
+    # via requests
+docutils==0.18.1
+    # via
+    #   myst-parser
+    #   sphinx
+    #   sphinx-rtd-theme
+idna==3.10
+    # via requests
+imagesize==1.4.1
+    # via sphinx
+jinja2==3.1.4
+    # via
+    #   myst-parser
+    #   sphinx
+markdown-it-py==2.2.0
+    # via
+    #   mdit-py-plugins
+    #   myst-parser
+markupsafe==2.1.5
+    # via jinja2
+mdit-py-plugins==0.3.5
+    # via myst-parser
+mdurl==0.1.2
+    # via markdown-it-py
+myst-parser==0.18.1
+    # via -r requirements.in
+packaging==24.1
+    # via sphinx
+pygments==2.18.0
+    # via sphinx
+pyyaml==6.0.2
+    # via myst-parser
+requests==2.32.3
+    # via sphinx
+snowballstemmer==2.2.0
+    # via sphinx
 sphinx==5.3.0
-sphinx-rtd-theme==1.2.0
+    # via
+    #   -r requirements.in
+    #   myst-parser
+    #   sphinx-copybutton
+    #   sphinx-rtd-theme
+    #   sphinxcontrib-jquery
 sphinx-copybutton==0.5.2
-myst-parser==0.18.1
+    # via -r requirements.in
+sphinx-rtd-theme==1.2.0
+    # via -r requirements.in
+sphinxcontrib-applehelp==2.0.0
+    # via sphinx
+sphinxcontrib-devhelp==2.0.0
+    # via sphinx
+sphinxcontrib-htmlhelp==2.1.0
+    # via sphinx
+sphinxcontrib-jquery==4.1
+    # via sphinx-rtd-theme
+sphinxcontrib-jsmath==1.0.1
+    # via sphinx
+sphinxcontrib-qthelp==2.0.0
+    # via sphinx
+sphinxcontrib-serializinghtml==2.0.0
+    # via sphinx
+typing-extensions==4.12.2
+    # via myst-parser
+urllib3==2.2.3
+    # via requests