Reorganize the project concept documentation (#9121)

- Adds a collapsible section for the project concept
- Splits the project concept document into several child documents.
- Moves the workspace and dependencies documents to under the project
section
- Adds a mkdocs plugin for redirects, so links to the moved documents
still work

I attempted to make the minimum required changes to the contents of the
documents here. There is a lot of room for improvement on the content of
each new child document. For review purposes, I want to do that work
separately. I'd prefer if the review focused on this structure and idea
rather than the content of the files.

I expect to do this to other documentation pages that would otherwise be
very nested.

The project concept landing page and nav (collapsed by default) looks
like this now:

<img width="1507" alt="Screenshot 2024-11-14 at 11 28 45 AM"
src="https://github.com/user-attachments/assets/88288b09-8463-49d4-84ba-ee27144b62a5">

docs/guides/integration/github.md

@@ -181,7 +181,7 @@ jobs:
 !!! tip
 
     The
-    [`UV_PROJECT_ENVIRONMENT` setting](../../concepts/projects.md#configuring-the-project-environment-path) can
+    [`UV_PROJECT_ENVIRONMENT` setting](../../concepts/projects/config.md#project-environment-path) can
     be used to install to the system Python environment instead of creating a virtual environment.
 
 ## Caching