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/jupyter.md

@@ -6,8 +6,8 @@ with a project, or as a standalone tool.
 
 ## Using Jupyter within a project
 
-If you're working within a [project](../../concepts/projects.md), you can start a Jupyter server
-with access to the project's virtual environment via the following:
+If you're working within a [project](../../concepts/projects/index.md), you can start a Jupyter
+server with access to the project's virtual environment via the following:
 
 ```console
 $ uv run --with jupyter jupyter lab
@@ -96,8 +96,8 @@ an isolated environment.
 ## Using Jupyter with a non-project environment
 
 If you need to run Jupyter in a virtual environment that isn't associated with a
-[project](../../concepts/projects.md) (e.g., has no `pyproject.toml` or `uv.lock`), you can do so by
-adding Jupyter to the environment directly. For example:
+[project](../../concepts/projects/index.md) (e.g., has no `pyproject.toml` or `uv.lock`), you can do
+so by adding Jupyter to the environment directly. For example:
 
 ```console
 $ uv venv --seed