Maintainers: rework pages

[ntarocco]

As discussed IRL with @slint, we should update some text in maintainers and governance sections.
Some of the following topics might have to be added to the InvenioRDM docs site instead.

• Add/rework maintainer section: we would like to write down what are maintainers responsibilities, and the procedure and criteria on how to become one (a new section "Onboard new maintainers"). We could take inspiration from some other famous projects.
• In the governance section, we might want to add the names related to the listed roles.
• The process of contributing via fill in the "project" template was not very successful: if it was documented, check what still makes sense. We might want instead point users to the README here.
• The People page is outdated.