Clarify documentation build instructions

Updated instructions for installing dependencies and building documentation.

Author: lauraporta

docs/source/contributing.md

@@ -108,15 +108,16 @@ please don't hesitate to make a contribution.
 The documentation is found in the `docs/source` folder, where the structure mirrors the rendered website.
 
 Dependencies for building the documentation locally can be found at `docs/requirements.txt`.
-To install these, change directory to the `docs` folder in your terminal and type:
+To install these, from the root folder run:
 
 ```
-pip install -r requirements.txt
+pip install -r docs/requirements.txt
 ```
 
-The command to build the documentation is:
+From the docs folder, build the documentation with `make`:
 
 ```
+cd docs
 make clean api_index.rst html
 ```