[website, docs] Use text description instead of GitHub logo

### Description

We've received some feedback that it isn't clear to everyone that the GitHub logo on the website and in the documentation book points to the source code for the project. We should consider replacing it with a more literal description such as 'Source Code' or perhaps just 'Code' to save space.

Locate the GitHub logo links in the website and documentation book, which are the two surfaces named in the issue. Replace the logo's ambiguous presentation with a literal label such as "Source Code" or "Code", then verify both locations clearly identify the source-code link.

Written by the indexing model from the issue text.