Change references to project name to path in Pages docs

What does this MR do and why?

Pages documentation refers to project name as a component of the URL, when it should actually refer to project path. This leads to confusion when users try to change the project name to change the location of the pages deployment. It doesn't work, and the documentation is misleading.

Moreoever, project name and path treat whitespace and case differently.

Related to Docs - product feedback: Confusing Instructions... (#385242 - closed)

Throughout this MR I have replaced usage of projectname with the following:

• project-slug when specifically referring to the part of the URL path representing the project.
• project-path when describing the path to a project in a pages deployment, which can consist of an artbitrary number of subgroups (or none). Valid examples are; subgroup/subgroup2/project-slug, subgroup/project-slug, and project-slug.

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