Docs/theme/MAINTAINERS: Modify/Update list of MAINTAINERS

So far it has been mostly my duty to create and maintain docs' design/theme.

This commit adds myself to the list of maintainers by modifying
the MAINTAINERS file under the docs/theme directory.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5439 from ostezer/docs-mkdocs-theme-maintainer-update

Docs/theme/MAINTAINERS: Modify/Update list of MAINTAINERS