Update link to theme documentation on project page

Problem/Motivation

The title of the link to the documentation is just the URL and it directs to the Drupal 7 documentation.

Steps to reproduce

Proposed resolution

Edit the 'Resource' section of the project page as follows
Title: Documentation
URL: https://www.drupal.org/docs/contributed-themes/seven →

Remaining tasks

User interface changes

API changes

Data model changes