Hello Dev,
Thanks for your interest in joining us! Welcome to the team! I'm replying your e-mail with copy to our e-mail list so everyone can welcome you as well.
You are absolutely correct, contributing to the documentation is a great opportunity to learn about the many features of LibreOffice Community.
As you mentioned, I believe you already have access to our Nextcloud system. If that is the case, you should already be able to see the documents related to Calc.
Check the following path: /LibreOffice Documentation/English/Calc Guide/7.1
You will see a few folders in there, currently we are working on the WIP (as in Work in Progress) folder. If you want to review a chapter, please let us know by filling the Guide Status.ods spreadsheet and by sending an e-mail to the list.
In case you do not have access to Nextcloud yet, please follow the steps described here: https://wiki.documentfoundation.org/Infra/SingleSignOn
If you have any question, feel free to ask in the list and someone will surely help you.
Best regards,
Felipe Viggiano