docs: Add requirements.txt file with dependencies for Sphinx.

Signed-off-by: Jos Verlinde <Jos.Verlinde@Microsoft.com>

docs/develop/gettingstarted.rst

@@ -245,7 +245,7 @@ that you use a virtual environment:
 
    $ python3 -m venv env
    $ source env/bin/activate
-   $ pip install sphinx
+   $ pip install -r docs/requirements.txt
 
 Navigate to the ``docs`` directory:
 

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx~=7.2.6
+sphinxcontrib.jquery==4.1