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/docker.md

@@ -161,7 +161,7 @@ RUN uv run some_script.py
 !!! tip
 
     Alternatively, 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 set before syncing to install to the system Python environment and skip environment activation
     entirely.
 
@@ -366,7 +366,7 @@ _contents_ are not copied into the image until the final `uv sync` command.
 
 !!! tip
 
-    If you're using a [workspace](../../concepts/workspaces.md), then use the
+    If you're using a [workspace](../../concepts/projects/workspaces.md), then use the
     `--no-install-workspace` flag which excludes the project _and_ any workspace members.
 
     If you want to remove specific packages from the sync, use `--no-install-package <name>`.