Adapt GitHub file path

[remibetin]

For the "Fork & Edit" link, the path to the file in GitHub is constructed as follows: https://github.com/{{repo}}/edit/{{branch}}/{{path}}

This PR clarifies what is assigned to path depending on the situation:

• By default, path = path to the file from the root of the Jekyll project.
• If the page is part of a collection AND the repository is w3c/wai-website, we add collections/ before the path to the file. Indeed, all collections are stored in a collections/ folder in this repo.
• If a github.path attribute has been manually set in the frontmatter of the page, it is used in priority.

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