Share code between playbook and accessibility manual

[yndajas]

This is an exploratory issue. Can we share code between the playbook and accessibility manual? This would help us maintain feature parity between the two sites

Git submodules are an option, but not one that many developers enjoy working with!

We might want to share the contents of the following folders, while allowing for customisation (e.g. to have a different colour scheme between the two sites; to use a Jekyll variable to populate the site name):

• https://github.com/dxw/playbook/tree/main/src/_includes
• https://github.com/dxw/playbook/tree/main/src/_layouts
• https://github.com/dxw/playbook/tree/main/src/_plugins
• https://github.com/dxw/playbook/tree/main/src/_sass
• https://github.com/dxw/playbook/tree/main/src/_webpack

We could also share these, as well as some of the root folder config files:

• https://github.com/dxw/playbook/blob/main/src/search.html
• https://github.com/dxw/playbook/blob/main/.github/workflows/lint.yml

Some of this would be duplicated, but copying some YAML but not all of it from other files might be painful:

• https://github.com/dxw/playbook/blob/main/src/admin/config.yml

This would close #1337, dxw/accessibility-manual#143, dxw/accessibility-manual#144, and dxw/accessibility-manual#145

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Note that we are open to exploring new platforms for the playbook in future, so we should be mindful of not overengineering a solution to the duplication and parity problem if it ends up being a short term one