Refine Advisory Board and TAG chair selection process

[torgo]

Removed redundant text regarding Advisory Board Chair selection and clarified the appointment process for Staff Contact. As per #1182. This is my first PR on the Process so please bear with me if the formatting is not what it should be.

Open issues on this PR:

• The current wording of the combined text says that the chair MUST be selected from the group in question, but this is not the case for the AB, so should this must actually be a SHOULD (opening up the possibility of an external chair for the TAG as well) or should there be separate language in here for TAG and AB?
• The AB chair selection process includes a separate ratification step by secret ballot: "Upon appointment, the Chair(s) are subject to ratification by secret ballot, requiring approval by two thirds of the elected participants." Should this be a part of the combined text, or do we leave it to each group to develop their own agreed way to measure the consensus decision?

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