Address top 404s for 26Q2

[chalin]

Description TBC

Tasks

• Enable Hugo's default robots.txt generation #9561 - this is the top 404. This is actually a valid path, we just didn't generate a default file. The PR fixes that: it does little harm to have the Hugo-default robots.txt file generated, but we now have advantage is that those 404s won't "polute" our data collection.
• Publish favicons at the root and drop Docsy icons #9563
• Will need to make followup changes after Theme should not have default favicons google/docsy#2595 is fixed and Docsy is updated (as is mentioned in the PR files)
• ...