[DOCS]: Add "Themes" page to branch 5.x

[adiati98]

Description

Whenever parent issue (#471) has completed, cherry-pick the PR commits (excluding merge base branch commits) to 5.x.
Refer to the Git cherry-pick section of the Community Handbook for detailed instructions.

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Important

Before the PR for the parent issue is merged, the status of this issue is blocked.

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Relevant Documentation URL

https://devdocs.mautic.org/en/5.x/rest_api/themes.html

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

No

Suggestions (Optional)

No response

Screenshots or Screen Recordings (Optional)

No response