Rename project to Antora Collector and repository name to antora-collector

To be consistent with the terminology used in the docs and community chat, and to align with Antora Assembler, rename the project to Antora Collector. The project will still provide the @antora/collector-extension package, but this change leaves the project scope open to make room for other extensions and tools provided by this project in the future.

Generally, we should name projects that provide an extension or extensions without the -extension suffix to make room for future expansion, such as introducing related or usage pattern-based extension or exporting reusable logic for the development of like extensions.

As part of this change, the docs for the extension should be moved to the docs/collector-extension folder in the source repository. The docs component name is still collector-extension as that is the user-facing piece of this project.