Membership: contact handling improvements

Some improvements to consider in the core contributor membership process:

• Add an additional question in the process: "what pronoums you would like to use?".
• Process may need more details in how to handle OpenPGP keys and signatures (whether TOFU could be accepted, key signing parties etc). Process says that "access to authenticated resources requires your OpenPGP key to be signed by at least one other Core Contributor", but nothing is said about the verification process.