Add script that will allow for a local test run of documentation and linting

Signed-off-by: Karel van Klink <karel.vanklink@geant.org>

docs/test_documentation.sh

+#!/bin/bash
+
+docker run -it --rm -v $(pwd):/docs sphinxdoc/sphinx:latest /bin/bash -c \
+"pip install sphinx_rtd_theme myst-parser;cd docs;make html"
+
+docker run -it --rm -v $(pwd):/gap jdkato/vale:latest --config="/gap/vale/.vale.ini" sync
+docker run -it --rm -v $(pwd):/gap jdkato/vale:latest --config="/gap/vale/.vale.ini" /gap/build/html/_sources/