fix: broken "core concepts" link in docs website (#940)

# What does this PR do?
The `core concepts` link on [this
page](https://llama-stack.readthedocs.io/en/latest/contributing/new_api_provider.html)
is currently broken - this PR fixes that link

## Test Plan
Ran local docs build as described
[here](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md#building-the-documentation)

## Sources
N/A

## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [x] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [x] Updated relevant documentation.
- [x] Wrote necessary unit or integration tests.

Signed-off-by: Nathan Weinberg <nweinber@redhat.com>

docs/source/contributing/new_api_provider.md

@@ -3,7 +3,7 @@
 This guide will walk you through the process of adding a new API provider to Llama Stack.
 
 
-- Begin by reviewing the [core concepts](../concepts/) of Llama Stack and choose the API your provider belongs to (Inference, Safety, VectorIO, etc.)
+- Begin by reviewing the [core concepts](../concepts/index.md) of Llama Stack and choose the API your provider belongs to (Inference, Safety, VectorIO, etc.)
 - Determine the provider type ({repopath}`Remote::llama_stack/providers/remote` or {repopath}`Inline::llama_stack/providers/inline`). Remote providers make requests to external services, while inline providers execute implementation locally.
 - Add your provider to the appropriate {repopath}`Registry::llama_stack/providers/registry/`. Specify pip dependencies necessary.
 - Update any distribution {repopath}`Templates::llama_stack/templates/` build.yaml and run.yaml files if they should include your provider by default. Run {repopath}`llama_stack/scripts/distro_codegen.py` if necessary.