[DOCS]: Add "Dynamic Content" page to branch 7.1

Description

Whenever parent issue (#427) has completed, cherry-pick the PR commits (excluding merge base branch commits) to 7.1.
Refer to the Git cherry-pick section of the Community Handbook for detailed instructions.

[!IMPORTANT]
Before the PR for the parent issue is merged, the status of this issue is blocked.

Relevant Documentation URL

https://devdocs.mautic.org/en/7.1/rest_api/dynamic_content.html

Are you willing to help us fix this by making a pull request?

No

Suggestions (Optional)

How to update the page

• Find the file that needs to be updated in the docs/ directory. The dev docs URL format is as such:

  https://devdocs.mautic.org/en/x.x/FOLDER_NAME/FILE_NAME.html
  

  For example, if the relevant URL is https://devdocs.mautic.org/en/7.0/plugin_extensions/manipulating_contacts.html, then find manipulating_contacts.rst file inside the plugin_extensions folder.

• Transfer the content under the note admonition. Look at other pages on how to structure the content.

• Review and update the content when you see fit.

• Follow the contributing guidelines that are listed in the "Resources" section above.

Screenshots or Screen Recordings (Optional)

No response

Wait for parent issue #427 to complete, then inspect the 7.1 docs path for rest_api/dynamic_content.rst and compare it with the referenced Dynamic Content page. Cherry-pick the relevant parent PR commits without merge-base commits, transfer the content under the note admonition, and verify the 7.1 documentation page builds correctly.