merged 3.18 and 3.19

[andreadavanzo]

I merged 3.18 "Include Files That Are Automatically Expected" and 3.19 "Use Plaintext Formats When Appropriate" in one point "3.18 Include Expected And Beneficial Files".These are the reasons:- robots.txt was mentioned in both sections, so merging the two avoids repetition.

• The two existing points were referring mainly text files (exclude favicon.ico)
• The title "Use Plaintext Formats When Appropriate" was not describing the context of the points referred

I believe that merging the two sections, the explanation becomes more comprehensive. It effectively combines the expected files (like favicon.ico, robots.txt, etc.) and the beneficial files (like ads.txt, carbon.txt, etc.) into one unified guideline, making it easier for web developers to understand the full scope of important files they should include on their websites.
The two criteria now are extended to any new web standards or specifications.
The Additional Content sections have been merged as well.

[AlexDawsonUK]

Discussed in Slack & Chat to merge the guidelines manually to deploy across all supplements.