[DOCS]: Link to Themes at developer docs points to old docs

[adiati98]

Description

We have a new link for developer documentation. But the link to themes developer documentation is still pointing to the old one.

Relevant Documentation URL

https://docs.mautic.org/en/5.2/themes/theme_structure.html

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

No

Suggestions (Optional)

Change the link from https://developer.mautic.org/#theme-directory-structure to https://devdocs.mautic.org/en/5.x/themes/getting_started.html

Screenshots or Screen Recordings (Optional)

No response