Additional instructions needed to build documentation

[gavin-k-lee]

Describe the bug
When building the documentation, one needs to add submodules when cloning.

To Reproduce
Steps to reproduce the behavior:

1. Follow instructions on: https://github.com/SwissDataScienceCenter/renku/blob/master/docs/how-to-guides/contributing/documentation.rst#L10
2. Run pipenv run make install
3. See error

PS C:\Users\leega\Documents\renku\docs> pipenv run make html
Running Sphinx v3.5.4
Initializing Spelling Checker 7.3.2

Extension error:
Could not import extension cheatsheet (exception: No module named 'cheatsheet')

Expected behavior
Documentation builds.

What needs to be added to dcumentation
One needs to run

git clone --recurse-submodules [email protected]:SwissDataScienceCenter/renku.git

when cloning the repo first!

Screenshots and/or execution output

Run environment (please complete the following information):

• Windows Powershell
• Python 3.9