[Doc] Capturing Git scm information when `conanfile.py` is in subfolder

Hi,

we are using the [Capturing Git scm information](https://docs.conan.io/2/examples/tools/scm/git/capture_scm/git_capture_scm.html) workflow to package our own Conan packages.
However, the use case as described in the documentation only works for the simple cases, where the `conanfile.py` is in the root of the git repository.

If not, it's difficult to get both the local development workflow via `conan build` and the cache workflow via `conan create` working simultaneously.
The solution we come up with looks like this:
```py
def layout(self):
# Needed for local developement workflow.
self.folders.root = "../../"
# But build folder etc. in the correct subfolder.
self.folders.subproject = "path/to/subfolder/in/repo"
cmake_layout(self)

# Set source to the git root, as the checkout via `source()` always clones the full repo.
self.folders.source = "."

def build(self):
cmake = CMake(self)
# As `self.source_folder` is the git root, we need to tell CMake where our "real" folder is.
cmake.configure(build_script_folder=self.folders.subproject)
...
```

* Is there a better way to achieve this?
* If not, I think this should be documented.

I feel like this case is not uncommon.
If you are having multiple Conan packages in a single git repository or don't have the Conan package at the root folder, this special handling is needed.

If you need to demonstrate the problem, you can use:
```
$ git clone https://github.com/conan-io/examples2.git
$ mkdir conan_capture_scm
$ cp -r examples2/examples/tools/scm/git/capture_scm conan_capture_scm/
$ cd conan_capture_scm/
$ git init
$ git add .
$ git commit -m WIP
$ cd capture_scm/
$ conan create .
$ conan build .
```

Start with the “Capturing Git scm information” page linked in the issue and reproduce the listed commands using the capture_scm example from examples2. Compare the conan build and conan create workflows when conanfile.py is in a subfolder. Done means either identifying a better supported approach or documenting the required layout and CMake handling for this case.

Written by the indexing model from the issue text.