Stackadapt Audience: add more details to the action destination doc

[Vanessa-SSY]

This pull request updates the documentation for the StackAdapt Audiences destination to improve clarity, update instructions, and fix formatting issues. The most significant changes are a complete rewrite and reorganization of the setup and audience sync instructions, clarification of mapping steps, and consistent use of external link formatting.

Documentation improvements and reorganization:

• Rewrote and reorganized the "Setting up the StackAdapt Audiences destination in Engage" and "Sync an Engage Audience" sections for better clarity and step-by-step guidance, including more explicit instructions for mapping and syncing audiences.
• Clarified the process for mapping fields and explained how mappings are shared across audiences, with instructions for creating new destinations if different mappings are needed.
• Updated instructions for creating a StackAdapt audience from an Engage audience, including corrections to navigation and terminology.

Formatting and consistency:

• Fixed formatting of external links throughout the document to use {:target="\_blank"} for consistency and proper rendering. [1] [2]
• Corrected formatting of info boxes and image references for improved readability.

Proposed changes

Merge timing

Related issues (optional)

[rchinn1]

Thanks for submitting this, @Vanessa-SSY! Left a few suggested changes for your review. After that, happy to re-review and merge this.

[github-actions]

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