Process Request: Introduce process for applying for Dex maintainership

Dex is used and loved by thousands. It should have a maintainer staff with the capacity to handle all this love.

I volunteered for maintainership and @sagikazarmark suggested first contributing a process for how people can become Dex maintainers.

I plan to use Flux's processes as reference to propose a process tailored for Dex's needs:

• https://github.com/fluxcd/community/blob/main/community-roles.md
• https://github.com/fluxcd/community/blob/main/PROCESS.md#applying-for-flux-maintainership

Any feedback from the community is welcome!