Provide example `.xlsx` file as a guide

[jacobcook1995]

At the moment, we only provide an empty template file in the documentation. This file is only provided in the data providers section of the documentation, when it really should be provided to data managers as well. The link to this file is also currently broken.

Beyond this it would be useful to include a populated example file, as some of the worksheets (particularly the data worksheet) are not intuitive to populate. This could be generated based on the examples currently given in the documentation.

We need to:

• [x] Fix the broken template link
• [x] Make sure that the template is linked in the data manager docs as well.
• [x] Create an example file with populated data and include it the docs

[jacobcook1995]

The PR above partly addresses the above however the following changes need to be made before this issue can be closed.

• [x] File download link should point to develop (once PR is merged there)
• [ ] File download link should point to main (once the release branch is merged)
• [x] Code in Summary.md should be simplified (once headers=False issue is fixed)