new Makefile target: docs (#6510)

* [doc] multiple corrections to "Summary of the tasks"

* add a new "docs" target to validate docs and document it

* fix mixup

CONTRIBUTING.md

@@ -165,6 +165,16 @@ Follow these steps to start contributing:
    $ make quality
    ```
 
+   If you're modifying documents under `docs/source`, make sure to validate that
+   they can still be built. This check also runs in CI. To run a local check
+   make sure you have installed the documentation builder requirements, by
+   running `pip install .[tf,torch,docs]` once from the root of this repository
+   and then run:
+
+   ```bash
+   $ make docs
+   ```
+
    Once you're happy with your changes, add changed files using `git add` and
    make a commit with `git commit` to record your changes locally:
 

Makefile

-.PHONY: quality style test test-examples
+.PHONY: quality style test test-examples docs
 
 # Check that source code meets quality standards
 
@@ -23,3 +23,8 @@ test:
 
 test-examples:
 	python -m pytest -n auto --dist=loadfile -s -v ./examples/
+
+# Check that docs can build
+
+docs:
+	cd docs && make html SPHINXOPTS="-W"