Adding further instructions for GitHub pages docs deployment in hook

[matt-graham]

Fixes #391

Adds some text to be printed out in post_gen_project.py hook when deploy_docs_to_github_pages cookiecutter option is True to give instructions on how to complete set up of deployment of HTML documentation on GitHub Pages.

Example output:

Initialized empty Git repository in /home/matt/projects/test-package/.git/
GitHub CLI detected, you can create a repo with the following:

gh repo create matt-graham/test-package -d 'A cookiecutter package with UCL ARC recommendations.' --public -r origin --source test-package

The 'Documentation' GitHub Actions workflow has been set up to push the built HTML 
documentation to a branch gh-pages on pushes to main for deploying as a GitHub 
Pages website. To allow the GitHub Actions bot to push to the gh-pages branch you 
need to enable 'Read and write permissions' under 'Workflow permissions' at

https://github.com/matt-graham/test-package/settings/actions

After the 'Documentation' workflow has successfully completed at least once you will 
also need to configure the repository to deploy a GitHub pages site from the content 
on the gh-pages branch by going to

https://github.com/matt-graham/test-package/settings/pages

and under 'Built and deployment' selecting 'Deploy from a branch' for the 'Source' 
drop-down and 'gh-pages' for the 'Branch' drop-down, leaving the branch path 
drop-down with its default value of '/ (root).

[paddyroddy]

Looks good 🫐

[samcunliffe]

Looks clear to me!