docs: CSS ?inline query parameter

[jackfranklin]

Description

I spent ages hunting for this feature to turn off CSS content injection
and then found the GitHub
commit
which I couldn't find mentioned in the docs at all, so I thought I'd add
a brief mention.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.