gsoc: add mentors to metadata, generate mentor list automatically

[vvolkl]

It'd be nice if we could fill the page https://hepsoftwarefoundation.org/gsoc/2025/mentors.html automatically with the information from the project proposals https://github.com/HSF/hsf.github.io/tree/main/_gsocproposals/2025 . Probably the easiest is to move the mentor information in the yaml header, similar to what is done with the project length/duration

[MohanadKh03]

Hello I have come up with a solution but there is a missing case that I am not sure how to handle
Some mentors do not have an organization name in the proposals so when they are added automatically to the mentors.html they would not have an organization name is that okay or how should it be handled ?
example:

[vvolkl]

Nice! The organization should in principle be mentioned in the proposal, it can just be missing in the list if not.

There should be no duplicated entries though.