Remove pkgdown config as_is = TRUE

[jamesmbaazam]

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Docs fix.

• What is the current behavior? (You can also link to an open issue here)

Some vignettes have pkgdown as_is = TRUE in their YAMLs. The reason was to allow the use of other features not provided by the HTML rendering format. See https://pkgdown.r-lib.org/reference/build_articles.html#output-formats.

• What is the new behavior (if this is a feature change)?

It removes the as_is = TRUE setting in the YAML headers of the vignettes as it reduces compatibility with incoming pkgdown versions.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information: