Add @Thels details to MAINTAINERS.md

[rocketstack-matt]

The MAINTAINERS.md file added in #2517 lists @Thels as a maintainer, but the Name, Organization, and Email fields are still placeholders:

GitHub Username	Name	Organization	Email
@Thels	please add name	please add organization	please add email

@Thels — please open a PR against MAINTAINERS.md to replace the placeholders with your details:

• Name: your full name
• Organization: your organization
• Email: your preferred contact email

Per the "Updating this file" section in MAINTAINERS.md, changes are made via PR, and help@finos.org should be emailed once merged.

Ref: #2517