Update Vanilla Content Guidelines

[Sophie-32]

Updated content guidelines in suggested mode to improve alt text guidance and match copy style guide - https://docs.google.com/document/d/12nLPp3ktElJCNl_i3q5fGOjWMPhPXXFsIJGH-AuxduQ/edit?tab=t.0

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/WD-32997.

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[diana-stanciulescu]

Decision:
Validated and needed asap, especially for accessibility guidelines.

[advl]

@Sophie-32 @diana-stanciulescu ETA 1 week